Drug analysis is an imperative activity to check the quality of a drug compound. Pharmacopoeial monographs provide important information about the quality of a drug substance. The expected quality of a medicine during period of use is also explained in such monographs. Analytical tools such as spectroscopic and chromatographic methods have been developed for such investigations. We have analysed the purity of a well known anxiolytic drug; diazepam, by using liquid chromatographic (LC) technique. It was noticed that with Zorbax Eclipse XDB – C8 (4.6 x 150 mm, 5 μm) column and recommended mobile phase comprising acetonitrile - methanol - potassium dihydrogen phosphate (22+34+44 v/v), the desired results obtained were not according the chromatograms provided by European Pharmacopeia (EP), but by using another column (ACE – 5 – C8) (4.6 x 150 mm, 5 μm), an extra peak of diazepam degradant was obtained, which showed that by using appropriate mobile phase containing CH3CN- CH3OH- KH2PO4 (20+32+48 v/v), the better results can be achieved. The mean retention time for diazepam analysis was 2.9 minutes.