Spatial pattern and spatial association are important methods to study population diffusion and community succession. Melica przewalskyi is a kind of perennial rhizomatous clonal herbs with strange taste,which is not edible for grazing animals. With the infuluence of climate change and overgrazing, Melica przewalskyi gradually replaced Stipa Krylovii and became a single dominant specie in the northern slope of Qilian Mountains.It is very important to understand their distribution patterns and spatial association in the process of patch formation and recession,which is critical for revealing the mechanism of the replacement of the dominant species and the relationship between Melica przewalskyi and Stipa Krylovii in the process.The experiments were conducted in Yue ya ai grassland (38°47'15.8″ N,99° 43' 35.9″ E) which is located in the upper reaches of Shi yang river in Sunan country Gansu province, China. In early August, 2010, four different patches in different recession stages, such as formation stage, diffusion stage, stable stage and recession stage, were respectively selected in the study area, and a sample with no patch was selected as control plot(CK). The measured value L(d) and the value of upper and lower envelopes were calculated by the Visual Fortran 6.5 based on the coordinate getting by GetData Graph Digitizer 2.22. Origin 7.5 was used as the plot software. This plot in the lower left corner coordinate origin to the intersection, using the step of 5 cm, the number of Monte_Carlo stochastic simulation is 20 times, dashed line are the 95% confidence envelops for the same process. The results showed that:the aggregation of Melica przewalskyi population was intensely decreased and the spatial distribution pattern transfered from clumped to random, Melica przewalskyi population showed clumped pattern at all scale in the no patch stage(CK) and the patch formation stage(A), while it showed clumped pattern at 0-90 cm, 0-70 cm and 0-40 cm and random pattern at 90-100 cm, 70-150 cm and 40-150 cm in the diffusion stage(B), stable stage(C) and recession stage(D). The aggregation of Stipa Krylovii population was intensely increased, the spatial distribution pattern transfered from random to clumped. Stipa Krylovii showed random pattern at all scale in the CK stage, while it showed clumped pattern at 0-47 cm, 0-75 cm, 0-130 cm scale and random pattern at 47-50 cm, 75-100 cm and 130-150 cm scale in the A,B and C stage, and in the D stage,it showed clumped pattern at all scale.The spatial association of Melica przewalskyi replacing Stipa Krylovii gradually transferred from negative to no association. Negative correlation increased significantly in the beginning, and then weakened.Territory density of Melica przewalskyi first increases, and then decreases.As to Stipa Krylovii,the territory density gradually reduce. Competition is a internal factor causing the formation and change of the population spatial distribution pattern. The population spatial distribution pattern changes with the intraspecific competition and the interspecific competition changing into each other. The distribution pattern tend to clumped as the interspecific competition was the mainly factor, while the pattern was random or regular as the form tend to intraspecific competition. The competition showed unsymmetry. When the change of Melica przewalskyi population pattern was mainly caused by interspecific competition, Stipa Krylovii population pattern could change owing to intraspecific competition or intraspecific competition. Negative correlation was very significant when interspecific competition tends to be predominant in both populations.