The effects of DG on the protection of radial distribution system are discussed. With MATLAB/simulink software it builds distribution system model and DG model and calculates the current protection, according to the size and location of DG the effects on the short circuit current and protection of distribution system. The result shows the size of DG affects protection and makes it lose selectivity probably, DG upstream protection makes it amplify protection range, DG downstream protection makes it reduce protection range, and the original protections are necessary to install directional element.