We study the effect of gamma-ray beaming on gamma-ray emission of the pulsars in a self-sustained outer gap model. In this model, averaged gamma-ray flux is a function of period, magnetic held, magnetic inclination angle and solid angle of gamma-ray beaming for a gamma-ray pulsar. We generate a sample of gamma-ray pulsars with their ages less than 10(6) years by using the Monte Carlo method, and then study the gamma-ray beaming effect. The comparison of distributions of periods, magnetic fields, distances, gamma-ray energy fluxes and period derivatives of the simulated gamma-ray pulsars with those of observed gamma-ray pulsars by the detector EGRET shows that gamma-ray beaming has an important role on the detection of gamma-ray pulsars. Furthermore, possible gamma-ray pulsars observed by the detector GLAST are predicted.