Metformin (MET) is a biguanide, the first line therapy for type 2 diabetes[1]. This study aimed to develop a capillary zone electrophoretic method for the stability study of MET. MET was forced to degrade under various stress conditions including alkaline, acid and neutral hydrolysis, oxidation and photolysis[2]. The capillary zone electrophoresis method was established to separate and quantify MET and its major degradation product (i.e. cyanoguanidine, (CGN). The optimum condition was in 40 mM citrate buffer (pH 6.7) using a fused-silica capillary with an effective length of 60 cm and an inner diameter of 50 µm, injection at 50 mbar for 5 s, temperature of 30 ºC, applied voltage of 15 kV and diode array detection (DAD) at 214 nm. MET and CGN were   separated at 9.9 and 19.0 min, respectively, with a resolution (R_s) of 38.9. Results revealed that MET was stable in neutral and acid (0.1 N HCl) hydrolysis, but degraded to CGN under alkaline hydrolysis (0.1 N NaOH) and oxidation (3% H₂O₂).  Elevated temperature and exposure of MET to sunlight accelerated the degradation of MET. Validation and application of the method are under investigation.