The parasitoid fitness, including spatial distribution, niche overlap, the parasitoid rate of Bracon nigrorufum Cushman and its response on population density of Etiella zinckenella Treitschke, was studied. The results showed that the distribution pattern of B. nigrorufum and E. zinckenella were similar not only in the woodland, but also in single tree of Caragana. Their aggregate levels and densities were highest in the top and south direction of the tree. The parasitoid rates of B. nigrorufum were higher in the top and the east direction. Negative nonlinear relationship showed in the rate of parasitism of B. nigrorufum and density of E. zinchenella. The parameters, such as niche width, niche overlap and coefficient of interspecific competition, were higher, which indicated that B. nigrorafum had strong ability to control E. zinchenella. none