Abstract: Based on the field Investigation of Downy Mildew of welsh onion, physiological analysis was done to both resistant and susceptible varieties. The results showed that the chlorophyll content and carotenoid content dropped on both varieties. But the susceptible cultivar dropped more than the resistant one when the varieties were infected by Downy Mildew. The soluble sugar content of both euhivars increased after infection, but the susceptible eultivar increased more than the resistant one. The soluble protein content in susceptible cultivar increased a little, and the resistant one dropped after infection. The free amino acid content and MDA of the susceptible cultivar increased more than the resistant one. The proline content in the resistant cuhivar increased more than the latter after infection. In normal condition, the POD activity and SOD activity in the resistant cuhivar were higher than those in the susceptible one. After infection, the resistant cultivar increased more than the latter.