An experiment was conducted using a resistant cultivar of wheat (Triticum aestivum L. Nannong 99-18) and a susceptible cultivar wheat ( Sumai No. 3) to investigate effects of silicon (Si) on activities of defenserelated enzymes (chitinase, β-1,3-glucanase, phenylalanine ammonialyase (PAL) and polyphenoloxidase (PPO) ) and microregional distribution of silicon in leaves infected by powdery mildew (Blumeriae graminis f. sp. Tritici). When wheat plants were not inoculated with powdery mildew, application of Si did not have much effect on activities of β-1, 3-glucanase, PAL and PPO in both wheat cultivars or on activities of chitinase in the susceptible wheat cultivar, however, Si application was found to increase the activity of chitinase in the resistant wheat cultivar. Moreover, the activities of chitinase, β-1,3-glucanase，PAL and PPO increased significantly in both wheat cultivars after infection of powdery mildew. Except for the activities of PAL and PPO in the resistant wheat cultivar, all enzyme activities were significantly higher in Si-treated plants than in non-Si-treated plants after wheat plants were infected by powdery mildew. In addition, regardless of inoculation and Si addition, Si was mainly concentrated in trichomes and vascular bundle tissues. However, the relative content of Si markedly increased in leaf tissues, and Si rapidly accumulated at infected sites when plants were infected by powdery mildew, especially in plants treated with Si. These findings suggest that Si enhances wheat resistance to powdery mildew not only by promoting expression and activities of defense-related enzymes, but also by building up physical barriers against pathogens through Si accumulation in plants infected by powdery mildew.