How to optimize ice storage fraction and determine ice making mode a common method of selecting capacity of chillers now. In view of this situation, a new kind of method on determining of ice making mode of chiller is introduced according to types of chiller and ice storage. Formula of capacity of chiller are given under the full storage, partial storage and chiller priority control, partial storage and ice priority control. The important conclusion that ice storage fraction is independent on capacity of chiller under specified control is obtained. It is very important to determine properly capacit of chiller and ice making mode in order to design reasonably ice storage system. An engineering example is discussed in detail.