The magnetoresistance of magnetic granular alloys has been studied based on a quantum phenomenological model.We take into account the phase interference effectamong the impurity atoms in a grain to calculate the scattering cross section between conduction electron and atom.According to the size distribution data provided by the experiment reported,we have derived an expression for the magnetoresistance depending on the grain size,size distribution,and applied magnetic field.The calculated results for the Fe-Ag sample have shown that the variation character of magnetoresistance with the external field depends on the size distribution of the grains.At a size scale D0,the magnetoresistance shows a maximum value.The size scale D0 depends on the applied field,the sample materials and fabrication conditions.