Abstract: OBJECTIVE To establish an HPLC method for the assay of mosapride citrate related substances in the capsule that might originate from synthesis processes or degradation. METHODS The related substances in mosapride citrate capsules were analyzed by a chromatographic system comprising a reverse phase C18 analytical column, a mobile phase of methanol-0.02 mol·L^-1 potassium dihydrogen phosphate buffer (pH adjusted to 4.0 with o-phosphoric acid) (50∶50), a flow rate of 1 mL·min^-1 and a UV detector set at 274 nm. RESULTS The HPLC method had shown good chromatographic resolution for mosapride citrate and its related substances. The analyte concentration was found to be 500 mg·mL^-1, and RSD of peak area responses of the related substances was 1.3%. The sample and standard solutions were stable for 24 hours at room temperature. The calibration curve was linear in the range from 3.0 to 15.0 mg·mL^-1 (r=0.999 7, n=6). The precision RSD at three different concentration levels (3, 5, 10 mg·mL^-1) were less than 1.5%. The recovery was between 100.5% and 101.4%. In addition, analysis of samples subjected to accelerated stability conditions showed that all degradants were resolved from the active component, resulting in a stability-indicating assay. CONCLUSION The HPLC method is simple, rapid, selective and suitable for quantitative determination of mosapride citrate related substances in the capsule.