To develop a HPLC assay for the determination of metformin hydrochloride-related substances.The separation was performed on SHIMADZU VP-ODS (250 4.6 mm, 5 microm) column. The mobile phase of dicyandiamide was composed of methyl alcohol-1 mmol x L(-1) sodium dodecylsulfate in 10 mmol x L(-1) phosphate salt solution (60:40) (pH=5.5). The mobile phase of other related substances was composed of methyl alcohol-1 mmol x L(-1) sodium dodecysulfate in 10 mmol x L(-1) phosphate salt solution (55:45)(pH=5.5). The detection wavelength was 232 nm, and the running speed was 0.8 ml min(-1) at room temperature.Good resolution of dicyandiamide and main peak was obtained. The test results were reproducible.The method is simple, rapid and suitable for the determination of dicyandiamide and other metformin hydrochloride-related substances.