The climatic characteristics of heavy fog in Urumqi in 1976-2006 w ere analyzed statistically. The results showed some features about heavy fog in Urumqi as follows: it mostly appeared in nighttime, the duration of most f og was within 3 hours. Over the past 31 years average fog days every year were 2 9.5d; it appeared from November in winter to March next year; the duration of heavy fog in winter was the longest. The fog days was decreasing gradually with the trend of collectivity in the past 31 years, its average decreasing rate was 1.9d/10 a. The fog days decreased obviously in 1990's, but increased rapidly in twenty first century. Fog occurred with the highest frequency when temperature, relative humidity, wind speed and air pressure were in 0～-10℃, 85%～95%, 0～3m·s -1 and 910～925hPa, respectively. The heavy fog in winter happened in the following con ditions of atmospheric stratification, such as lower, thicker and stronger inver sion layer. 