Abstract: A field plot experiment was conducted to evaluate effects of application amount of RhizoVital?42fl microbial agent on growth and downy mildew disease occurrence of Brassica rapa L. The results showed that RhizoVital?42fl microbial agent could not only reduce downy mildew disease occurrence of Brassica rapa L., promote growth of Brassica rapa L. and increase the yield, but also reduce the population of fungus in soil and improve soil microbial flora. And the RhizoVital?42fl microbial agent 1.80 L/hm2 treatment was the best of all the treatments, its control efficiency against downy mildew disease occurrence of Brassica rapa L. was up to 64.37%, and plant height increased by 5.70%, length and width of maximum Leaf by 7.62% and 5.62%, the crown width by 3.13%, number of productive leaves by 5.74%, the net-head-rate by 2.20%, root wet weight by 13.73% and yield by 9.02% . In a word, RhizoVital?42fl microbial agent reduced downy mildew disease occurrence of Brassica rapa L., promoted growth of Brassica rapa L. and improved soil microbial flora.