The purpose of this paper is to study numerically the characteristics of the indoor air flow of a square ceiling type diffuser according to vane angle and flow rate. The CFX of ANSYS 13.0 was used for the CFD tool. The size of room is 6m(X)×6m(Y)×2.7m(Z). Exhaust diffuser was arranged in diagonal direction to the supply diffuser. This diffuser was designed to have many holes, so supply air have long throw pattern with low velocity decay. The characteristics of the indoor air flow is studied for volume flow rate of 180CFM and 260CFM, and vane angle from 30° to 60°, each 10°.