The photostability of vitamin K1 (phylloquinone) in bulk powder and in its dosage form was studied according to the conditions suggested by ICH Guideline (option 2). The results showed that the rate of photodegradation of vitamin K1 follows first order kinetics and it is higher in its dosage form than in bulk powder. Attempts have been made to improve the photostability of vitamin K1 by combination with different photoprotective agents such as p-aminobenzoic acid (PAPA), tartaric acid, boric acid, citric acid, sodium benzoate, titanium dioxide (TiO2), zinc oxide (ZnO), propyl 4-hydroxybenzoate and methyl 4-hydroxybenzoate). The photodecompositions products were monitored by UV-Vis spectrophotometric method, HPLC. It was found that 0.01% PABA was the best photoprotective agent, which improved the photostability of vitamin K1 by 46 fold. The method was validated according to ICH guideline. The calibration graph was linear over the concentration range of 1.1× 10-5- 30.0 ×10-5 mol L-1. The limit of detection and the limit of quantification were 2.42 × 10-6 and 7.35 ×10-6 mol L-1, respectively.