Column Family has been used in NoSQL Database, and we have to think over how to divide the Column Family in the designing of NoSQL Database. But the dividing of column family is similar to the vertical partitioning in the distributed database. And there are some object functions of vertical partitioning in the database system, but they do not fit to the NoSQL Database. In this paper we will propose an objective function for dividing class family in NoSQL database. The objective function derived generalizes and subsumes earlier work on vertical partitioning in distributed database and the scenarios of NoSQL database. Furthermore, the approach proposed in this paper is shown to be useful to evaluate the cost of dividing column family.