Phase compositions and crystal structures of Gd6X2Si3 (X = Ni, Co) and Ln6Co2Si3 (Ln = Pr, La) have been studied. The magnetic properties of Gd6X2Si3 (X = Ni, Co) and Ln6Co2Si3 (Ln = Pr, La) have been evaluated from magnetization measurements performed by a superconducting quantum interference device magnetometer in a temperature interval of 5–400 K, and at magnetic fields up to 5 T. The crystal structures of Gd6X2Si3 (X = Co, Ni) and Ln6Co2Si3 (Ln = Pr, La) were found to be hexagonal at 300 K. The Curie temperature (TC), effective and magnetic moment of the compounds (at 5 K) have been determined for Gd6X2Si3 (X = Ni, Co), and Ln6Co2Si3 (Ln = Pr, La). TC was found to depend on composition, and reaches maximum value of about 300 K for the Gd6X2Si3 system. The magnetocaloric effect (magnetic entropy changes and relative cooling power (RCP) was found to depend on X and Ln, and the maximum RCP values were found to be larger than 500 J/kg near room temperature for the Gd6Ni2Si3 and Gd6Co2Si3 compounds. This value of RCP is comparable to the prototype magnetic refrigeration material, Gd.