Abstract: Three medium-entropy intermetallic compounds (Sm_1/3Ce_1/3Pr_1/3)Co₅, (Sm_1/3Ce_1/3Nd_1/3)Co₅ and (Sm_1/3Pr_1/3Nd_1/3)Co₅ and a high-entropy intermetallic compound (Sm_1/4Ce_1/4Pr_1/4Nd_1/4)Co₅ were designed on the basis of SmCo₅. The possibility of forming single-phase structure was predicted using atomic size difference and mixing enthalpy. Four intermetallics were synthesized via the vacuum arc-melting technology. The X-ray diffractometer (XRD), the energy dispersive X-ray spectrometer (EDS) and the vibration sample magnetometer (VSM) were used to study the phase, chemical compositions and magnetic properties. The results show that four intermetallics are single-phase structures and crystalline in a hexagonal CaCu₅ structure with a space group of P6/mmm. The rare earth atoms occupy 1a site. The atomic concentration at the rare earth site is equiatomic ratio. The room temperature magnetizations follow the Langevin model. Magnetization depends on the composition of the compound. The magnetic valence model was employed to check the valence of Ce. The calculated results show that Ce is tetravalent in (Sm_1/3Ce_1/3Pr_1/3)Co₅, (Sm_1/3Ce_1/3Nd_1/3)Co₅ and (Sm_1/4Ce_1/4Pr_1/4Nd_1/4)Co₅ and makes no contribution to the magnetic moment.