We have studied experimentlly the transition phenomenon of a granular flow from dilute flow to dense flow in two-dimensional channels. In the dense flow the outflow rate (Q) depends only on the opening (D) given by Q=aD3/2.For different inflow rates Q0 in a dilute flow, the critical opening-size with the channel relation of width (W) is given; and in a larger inflow, the critical opening-size is opproximately proportional to the square root of the inflow rate,Dc∝Q1/20.