This is a Phase II single center, open-label, single arm study in patients with advanced thymic epithelial tumors after failure of cisplatin-based combination chemotherapy. Patients will be treated with Pembrolizumab 200 mg every 3 weeks.

Inclusion Criteria:

1. Be willing and able to provide written informed consent/assent for the trial.
2. Be 18 years of age on day of signing informed consent.
3. Histologically proven thymic epithelial tumor (TET) patients
4. Inoperable or metastatic disease
5. More than one previous chemotherapy including at least one platinum-based regimen
6. At least one measurable lesion based on RECIST 1.1
7. Patients who could submit at least one unstained slide to evaluate the PD-L1 expression status (PD-L1 status, which is positive (expression > 1percent of tumor cells) or negative, is the prerequiste for the enrollment. If the submitted slides are unacceptable for the analysis for PD-L1 and there is no remained slide, the patient cannot be enrolled)
8. Have a performance status of 0 or 2 on the ECOG Performance Scale.
9. Demonstrate adequate organ function as defined in Table 1. Laboratory test must be performed within 10 days before date of treatment.
10. Female subject of childbearing potential should have a negative urine or serum pregnancy within 72 hours prior to receiving the first dose of study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required.
11. Female subjects of childbearing potential should be willing to use 2 methods of birth control or be surgically sterile, or abstain from heterosexual activity for the course of the study through 120 days after the last dose of study medication. Subjects of childbearing potential are those who have not been surgically sterilized or have not been free from menses for > 1 year.
12. Male subjects should agree to use an adequate method of contraception starting with the first dose of study therapy through 120 days after the last dose of study therapy.

Exclusion Criteria:

1. Is currently participating in or has participated in a study of an investigational agent or using an investigational device within 4 weeks of the first dose of treatment.
2. Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial treatment.
3. Has a known additional malignancy that is progressing or requires active treatment. Exceptions include basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or in situ cervical cancer that has undergone potentially curative therapy.
4. Has an active automimmune disease requiring systemic treatment within the past 3 months or a documented history of clinically severe autoimmune disease, or a syndrome that requires systemic steroids or immunosuppressive agents. Subjects with vitiligo or resolved childhood asthma/atopy would be an exception to this rule. Subjects that require intermittent use of bronchodilators or local steroid injections would not be excluded from the study. Subjects with hypothyroidism stable on hormone replacement or Sjorgen's syndrome will not be excluded from the study.
5. Has evidence of interstitial lung disease or active, non-infectious pneumonitis.
6. Has an active infection requiring systemic therapy.
7. Is pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the pre-screening or screening visit through 120 days after the last dose of trial treatment.
8. Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies).
9. Has known active Hepatitis B (e.g., HBsAg reactive) or Hepatitis C (e.g., HCV RNA [qualitative] is detected).
10. Has received a live vaccine within 30 days prior to the first dose of trial treatment.
11. Uncontrolled systemic illness such as DM, CHF, unstable angina, hypertension or arrhythmia
12. Has known active tuberculosis which was not treated
13. Has a known history of hypersensitivity to pemborolizumab
14. Has a treatment history wih mAb within 4 weeks prior to the first dose of trial treatment
15. Has had radiotherapy or chemotherapy within 14 days of the first does of trial treatment. Subjects who received radiotherapy >14 days prior to randomization must have completely recovered from any therapy related AEs/toxicities except peripheral neuropathy less than grade II
16. Has a known history of psychosis or substance abuse