Objective: 6-Mercaptopurin is antineoplastic drug that is included in antimetabolite group and is used in acute lymphoblastic leukemia medication. 6-Mercaptopurin is inactive pro-drug that will be metabolized into metabolites. One of its metabolites is 6-methylmercaptopurine. This study is aimed to optimize the analytical conditions and perform validation for the analysis of 6-mercaptopurine and 6-methylmercaptopurine in plasma. Method: Separation was performed using Waters 2996 HPLC, C18 Sunfire™ column (5μm, 250 × 4.6 mm) with the mobile phase containing water-methanol-acetonitrile with gradient elution, and detected at 303 nm. 5-Fluorouracil was used as internal standard. Plasma extraction was done by liquid-liquid extraction using dichloromethane. Result: The method was linear at concentration range of 2.0 - 200.0 ng/mL with r > 0.9991 for 6-mercaptopurine and 20 - 2000 ng/mL with r > 0.9993 for 6-methylmercaptopurine. Accuracy and precision within-run and between-run fulfill the acceptance criteria with % RE and relative standard deviation (% RSD) ≤ 20% (LLOQ) and ≤ 15% (QC samples). 6-Mercaptopurine and 6-methylmercaptopurine was stable in plasma at least for 21 days when stored at -20°C. Conclusion: The bio-analytical method was sensitive, selective and all the parameters fulfilled the acceptance criteria of the EMA Bio-analytical Method Validation Guideline, 2011.