This is an open-label, phase 2 study of KN026 in subjects with HER2 expressing gastric/gastroesophageal junction cancer to evaluate efficacy and safety. The subjects will receive KN026 10 mg/kg QW or 20 mg/kg Q2W or 30mg/kg Q3W until progressive disease, unacceptable toxicity or death.

The study consists of two arms as HER2 overexpression arm and HER2 expression arm and both arms will receive KN026 treatment.

Inclusion Criteria:

• Signed inform consent form(ICF)
• Age ≥ 18 years and ≤ 75 years, male or female
• Histologically or cytologically documented advanced gastric/gastroesophageal junction cancer HER2 overexpressing: IHC 3+ or IHC 2+ & ISH+ HER2 expressing: IHC2+ & ISH- or IHC 1+ & ISH+
• Received at least one prior standard therapy
• At least one evaluable lesion according to Response Evaluation Criteria In Solid Tumors（RECISIT） v 1.1
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
• Adequate organ function
• LVEF≥ 50% (ECHO)
• Female patients and males with partners of childbearing potential should be using highly effective contraceptive measures (failure rate of less than 1% per year). Contraception should be continued for a period of 24 weeks after dosing has been completed.
• Ability to comply with treatment, procedures and pharmacokinetics (PK) sample collection and the required study follow-up procedures

Exclusion Criteria:

• Accepted any other anti-tumor drug therapies within 4 weeks before fist dose
• Accepted radiotherapy within 4 weeks before enrollment
• An anthracyclines antibiotic treatment was received exceeding 320 mg/m² or other equivalent dose antharcyclines
• Subjects are eligible with clinically controlled and stable neurologic function >= 4 weeks, which is no evidence of CNS disease progression; Subjects with spinal cord compression and cancerous meningitis are not eligible
• Pregnant or nursing females；or intend pregnancy within this study period or within 6 monthes after the end of this study
• History of immunodeficiency, including HIV positive or other acquired, congenital immunodeficiency disease, or a history of organ transplantation
• Severe chronic and active infection, need to system antibiosis/antiviral treatment
• Cavity effusion (pleural effusion, ascites, pericardial effusion, etc.) are not well controlled, and need locally treatment or repeated drainage
• Even with peripheral or central venous nutritional support, unintentional weight loss ≥5% within 1 month before the first medication