The continued high temperature weather (CHTW) appeared in Beijing since June 23 1999. The measurements showed the PM2.5 mass concentrations were 2 to 3 times of that in non-CHTW period. However the diffusion conditions were still fine in these days. Both the highest hourly concentrations of O3 and the SO4(2-) composition in PM2.5 can identify that the photochemical action was very active. It is the very reason to produce more fine particles in atmosphere.