The electrical resistivity ( ρ ) and magnetoresistance of polycrystalline pellets of La1-xCaxMnO3 were measured in the 10—300K range in magnetic fields up to 1.8T. It was found that when x increases, both the peak of the electrical resistivity and the peak of magnetoresistance occurs at lower temperatures.The authors suggested that the change of x results in the change of proportion of Mn4+ ions.Variation of the proportion of Mn4+ ions leads to the change of magnetic structure of La1-xCaxMnO3.Zener's model of double exchange coupling can be used to explain the experimental results.