Experiment was conducted to study the biochemical changes in mustard in response to powdery mildew disease (Erysiphe polygoni. DC). Mustard cultivars viz., Skm-9804(V1, Medium susceptible), Skm-9801 (V2, Medium susceptible), GM-1, (V3, Susceptible), Varuna (V4, Highly susceptible) and Skm-9818, (V5, Highly susceptible) were grown under field condition in two plots. One plot was treated with fungicide Hexaconezole @ 1ml/lit i.e control plot while other was kept free for natural infection of powdery mildew disease in mustard plants i.e diseased plot. Leaf tissues were collected at pre infectional, infectional and post infectional stage from both plots and analysis were done with the biochemical parameters viz. total chlorophyll, soluble sugars, reducing sugars, non reducing sugars, crude protein, total phenol, epicuticular wax, lignin content and enzyme activity of peroxidase, polyphenol oxidase and cellulase. At harvest yield attributes were measured from both the plots. Results showed wide variation in healthy and diseased plants.