The compounds of SmCo5 and Sm0.8Tm0.2Co5.2 have been studied via X-ray diffraction (XRD) and magnetic measurements, in order to clarify the effect of Tm doping on the crystal structure and intrinsic magnetic properties of the SmCo5 compound. XRD results show that both compounds have a single hexagonal CaCu5 type crystal structure, and the Tm doping leads to an expansion of the hexagonal unit cell in the direction of c axis and a contraction of a, b axes and the cell volume. The Curie temperatures (TC) of the SmCo5 and Sm0.8Tm0.2Co5.2 are 957 and 965 K, respectively. Magnetic measurements show that both the magnetic anisotropy field (HA) and the magnetization at an applied field of 7 T (M7T) of SmCo5 are higher than those of Sm0.8Tm0.2Co5.2 at room temperature (300 K). However, the HA and M7T of SmCo5 become lower than those of Sm0.8Tm0.2Co5.2 with the temperature increasing to 473 K. Therefore, it is concluded that Tm doping in SmCo5 can effectively improve the magnetic properties at elevated temperature