The Sm substituting effects for La in La0.67Ca0.33MnO3 have been studied systematically. With increasing Sm doping amount, the metal-isolator phase transition temperature for the samples decreases monotonically, the corresponding peak resistivity increases dramatically and the Curie temperature decreases monotonically. The substitution of La-Ca-Mn-O with 0.13% Sm for La improved the magnetoresistance ratio by an order of magnitude. The effects of doping with Sm can be explained in terms of the lattice effects. An irreversible MR behaviour was observed in the Sm-doped compound. This effect was enhanced with increasing Sm doping amount.