【Objective】Biochar is a carbon-rich product of pyrolysis of biomass and can be used to ameliorate soiland promote crop growth.Recently,studies show that biochar has some good effects on control of soil-borne diseases. Researches in the past already demonstrated that straw biochar could effectively control Phytophthora blight of chilli pepper, through improving soil physic-chemical properties and microbiological properties, but little has been reported on how application rate affectsthe effect ofbiocharcontrolling the disease of Phytophthoracapsici.【Method】In this study, a pot experiment, designed to have 5 treatments concerning application rate, i.e. Treatment 0, 1, 2, 3 and 4 (0%, 0.33%, 0.66%, 1.33% and 2% w/w, respectively), was carried out to investigate effect of straw biochar controlling Phytophthora blight of chilli pepper, relative to application rate of biochar.The soils in the pots were analyzed separately for chemical properties (soil pH, electrical conductivity, organic matter, ammonium N, nitrate N,available Pand readily available K) and soil micro-biological properties (soil enzyme activity, microbial population, microbial community structure and soil microbial carbon utilization features).Soil microbial populationwasdetermined with the dilution plating method and real time PCR; soil microbial community structurewith PCR-DGGE andsoil microbial carbon utilizationfeatures with the Biolog system. Correlation analysis was done of biochar application rate, soil properties and disease control effect.【Result】The disease controlling efficiency in Treatment1, 2, 3 and 4 was -9.8%,8.6%,56.7% and 35.1%, respectively, forming an inverted U-shape curve. So the application rate of 1.33% (Treatment 3) was the optimal for disease control.With increasing biochar application rate, soil pH and organic matter content increased gradually, electrical conductivity, available P content and readily available K content did significantly, but ammonium and nitrate nitrogen contents did only slightly.Urease and β-glucosidase activities gradually decreased, FDA Hydrolase and sucrase activities increased firstand then decreased,total bacteria, total fungi and four groups of functional microorganisms gradually increased in population, and P.capsici first increased and then decreased in population with increasingapplication rate of straw biochar.Microbial metabolic ability, microbial diversity and microbial community evenness all displayed an inverted U-shape curve in response to the varying application rate of straw biochar, with peaks appearing all in Treatment 3(1.33%).DGGE profiles show that biochar at low application rates (0.33% and 0.66%) had little effect on microbial community structure, but when at high application rates (1.33% and 2%) it did significantly alter microbial community structure, significantly increasing the abundance ofPseudomonas, Sphingomonas, Ascomycota and Basidiomycota.【Conclusion】This study demonstrates prominently the importanceofapplication rate of strawbiochar in disease control.The control efficiency increases as application rate of straw biochar increaseswithin a certain range, which may be attributed to the function of biochar to improve soilchemical and biological properties.But when the application rategoes beyond the range, straw biocharcould weaken in control efficiency, which may be explained by the fact that too much biochar has some negative influences on soil carbon and nitrogen recycling, microbial activity and diversity.In addition,the effect of biochar controlling Phytophthora blight of chilli pepperis not much related to the population of P.capsici in the soil.