We study numerically the stability of granular flow on a rough slope in collisional flow regime in the two-dimension. We examine the density dependence of the flowing behavior in low density region, and demonstrate that the particle collisions stabilize the steady flow above a certain density in the parameter region where a single particle would show an accelerated behavior. Within this collisional steady flow regime, however, the uniform steady flow is only metastable and is shown to be unstable against clustering when the particle density is not high enough. (107kb)