This paper presents a derivation of monostatic radar cross section (RCS) of reflectarray. In 5G, the reflectarray and reconfigurable reflectarray have gained attention to reduce the coverage holes. The reflectarray is evaluated by the bistatic RCS measurement. However, the bistatic RCS measurement is relatively complicated as compared to monostatic RCS measurement. This paper proposes the theoretical equation of monostatic RCS of reflectarray is derived to predict bistatic RCS of the reflectarray. By using these equations, the relation between the design reflection angle of bistatic RCS of the reflectarray, and the monostatic RCS of the reflectarray. The derived equations is validated by the comparison of the simulation which is the reflecarray operating at 28 GHz.