Indian mustard [Brassica juncea (L.) Czern & Coss] is an important oilseed crop. Powdery mildew of mustard incited by Erysiphe cruciferarum (Opiz ex Junell) is the most important disease that causes a maximum reduction in yield and quality of mustard seed. The resistance of plants to various pathogens depends on the synthesis and level of various defense enzymes like hydrolyses peroxidases, phenol and PR-Protein. The present study focused on powdery mildew and its biochemical correlation with TSS, PR-Proteins and phenols content. The field studies observed that this disease incidence data were correlated with biochemical changes and levels of TSS, PR-protein and phenols activities. Among biochemical basis of resistance in mustard, TSS and Protein are maximum in healthy plants compared to infected plants, and Phenol content is minimum in healthy plants compared to infected plants were observed in leaves of infected plants with Erysiphe cruciferarum compared to the healthy ones. Biochemical changes in total soluble sugars, protein and phenol content were played a significant role in imparting resistance against this disease. A similar trend in these biochemical was also observed at tender and maturing stages of leaves.