By means of the data on athletes' performance in Beijing International Marathons from 1981 to 2003 and the daily or hourly meteorological data during the same time period,the relationships between athletes' performance in the marathons(both men and women) and weather conditions were analyzed quantitatively.The results show that weather factors have a great influence on the athletes' performance in the marathons: about 9 to 12 minutes.The correlation between temperature and athletes' performance is significant.The higher the air temperature is,the worse the performance.Drizzle benefits for acquiring better results.The comfort index is a good indicator of marathon performance for men athletes,and the wind-chill index for women.