The monitoring of PM2.5 conducted in Hangzhou City during the period of 2004-2005 shows that PM2.5 concentration changes with season as well as with location: higher in winter than in summer, and higher in downtown area than in well-afforested area. In addition, ratio of PM2.5 / PM10 is analyzed, ranging from 0.50 - 0.78, which indicates the urban people are more vulnerable to the PM pollution as PM2.5 is regarded being more harmful to health of human being.