A gradient reverse-phase liquid chromatographic (RP -LC) method was developed for the determination of related substances in Simvastatin drug substance. Simvastatin is a potent competitive inhibitor of HMG-CoA reductase, the rat e-limiting enzyme in cholesterol biosynthesis, can be used in treatment of hyperchol esterolemia. Good chromatographic separation was achieved for all the listed known im purities along with some new process impurities from the peak of interest. The current U SP HPLC method for the determination of related substances of simvastatin is not able to we ll separate some critical impurities that may form during the synthetic process or carried over f rom the starting materials. The chromatographic separation achieved on a ZORBAX col umn. The LC method employs solutions A and B as mobile phase. The solution A c ontains a mixture of phosphate buffer with pH 3 and Acetonitrile (50:50, v/v) and solutio n B contains pure Acetonitrile and the detection wavelength used 238 nm. The resolution be tween simvastatin and its closely eluting potential impurity, namely methyl ester of simvasta tin was found to be greater than 2. Forced degradation studies were performed for Simvastatin bulk drug using acidic condition, basic condition, oxidation, heat and light. All the degra dants products are well resolved in the developed method. The method was validated with res pect to linearity, accuracy, precision, robustness and forced degradation studies prove the stability-indicating power of the developed method.