To enhance the performance of the bank-data warehouse and reasonably determine the detail of the data and the data level about the warehouse,a data granularity partition of bank-data warehouse is presented.Firstly the granularity model of data warehouse is analyzed,and then the strategy of granularity partition is introduced.Based on the strategy a method of estimating a data warehouse size is put forward.On the basis of the principles of designing data granularity,a data granularity partition of bank is shown.This partition method effectively solve a most important design question faced by bank-data warehouse exploiter,and cause the other aspects of design and realization can carry on smoothly.