Abstract: Based on the conventional observation data and reanalysis data (horizontal resolution 1°×1°) from NCEP, two snow processes (rain to blizzard and heavy snow ) in the middle ten days of February 2009 were analyzed. Circulation pattern, influencing system, water vapor and dynamic conditions together with thermal structure of two processes were compared. The results show that two processes are different in many aspects. Both occur under stable Ural blocking high and are influenced by eastward mesoscale low pressure system in middle latitude. However, weather situation of rain to blizzard process is two ridges and one trough in middle and high latitude. The short wave trough combines with the south low trough and moves eastward with cold air together, and they converge with low level jet over Liaoning province. Weather situation of the heavy snow is low in the east and high in the west, and cyclone fluctuates eastward in middle latitude. Shear line moves northward and influences Liaoning regions together with warm and moist air current from the southwest. Two processes happen in where air pressure is below 600 hPa and relative humidity above 80%, and both are of low level convergence and high level divergence as well as deep ascending motion. Compared with the heavy snow process, the rain to blizzard process is of high water vapor contents and deep and strong convergence as well as large vertical speed. There is the obvious wind shear in two processes. The rain to blizzard happens when vertical wind shear increases rapidly. However, wind shear is stable in the heavy snow process. For the rain to blizzard process, precipitation strengths with the development of moist potential vorticity, and there is a good corresponding relationship between them. However, for the heavy snow process, moist potential vorticity is weak. Air pressure on 0 ℃ layer reaches 850 hPa in front of trough, and temperature in each layer descends rapidly and below 0 ℃ behind trough in the rain to blizzard process, while temperature of the total layer always is below 0 ℃ in the heavy snow process.