The purpose of this study is to compare effects of erlotinib and pemetrexed as maintenance therapy for lung adenocarcinoma. Primary Outcome is the progression free survival (PFS). Based on prediction of different potential benefit groups, this study may help the oncologist to optimize and improve the maintenance therapy plan for lung adenocarcinoma.

Inclusion Criteria:

• Histologically or cytologically diagnosed stage III/IV Lung Adenocarcinoma;
• Age of 18-75years; Gender Not Required;
• Received systemic clinical examination, including chest X-ray, abdomen B ultrasound, head CT scan, radionuclide bone scan;
• Received cisplatin chemotherapy for 4 cycles before this study, the curative effect of chemotherapy is SD or above SD (efficacy evaluation time is 21 days, if it is SD, then the longest diameter of target lesions shall not increase);
• Adequate hematologic, renal, and hepatic function, Specific index as follows: liver function: S-Bilirubin ≤1.5 ULN ; Transaminase≤2 ULN. renal function: S-Creatinine ≤1.2 ULN; blood urea nitrogen ≤1.2 ULN . ULN: upper normal limit. Marrow Hemopoietic Function: WBC≥4.0×10^9/l, ANC≥2.0×10^9/l platelet count ≥100×10^9/l, Hb≥100 g/l;
• ECOG PS 0-2,Life expectancy ≥ 3 months; endure more than two cycle chemotherapy;
• The patients have explicit lung tumor lesions and the lesions were measurable; (According to the standard of RECIST1.1, they should have at least one of accurately measurable lesions with the largest diameter ≥ 10mm by spiral CT, MRI);
• No history of serious drug allergy;
• Informed consent should be obtained before treatment.

Exclusion Criteria:

• Not histologically or cytologically diagnosed as Lung Adenocarcinoma
• The age of >75 years or <18 years.
• Major organ dysfunction and Serious Heart Disease (congestive heart-failure, incontrollable high-risk arrhythmia, unstable angina, alular disease, myocardial infarct and Resistant hypertension,);
• Serious complications and investigator consider it is unsuited enrolling;
• Pregnant or lactating women;
• Allergic to research drug;
• Received cisplatin chemotherapy for 4 cycles before this study, the curative effect of chemotherapy is PD