Abstract: Theoretical analyses on three clustering procedures show that:being influenced by the group size of merger at any step of fusion,Wards method gives the priority to the merger with less observations,and finally produces clusters of close size.On the contrary,centroid method gives the priority to the merger with more observations,and finally produces a huge cluster and many small clusters some of which contain only one observation.Average linkage method is almost not influenced by the group size and produces cohesive clusters with close variance.It duplicates climatic reality with the greatest accuracy.A regionalization of year-to-year variation in summer precipitation in eastern China verifies above mentioned conclusions.