Abstract:The micro-physical structure of precipitable stratiform clouds are analysed in the westerly trough system over the central and south parts of Hebei Province on April 16,1991. The results indicate that the development of precipitation was carried out mainly in the stratiform clouds in the middle and upper levels. Although there are favorable conditions for ice crystal growth i.e. the catalyzables of supercooled area and water. The development of precipitation is disadvantageous due to the thin water supply clouds and the high cloud base. After the aircraft seeding operation,the micro-structure of the clouds has been varied (the water content of cloud droplets and rain droplets increased obviously. There is an evident expanding in the droplets spectrum. etc. ),the precipitation capacity increased in the area of seeding operation and influenced area within the effective period of artificial catalysis. These results are in agreement with the clouds modification.