Objective To investigate the effect of chronic compression on the rat sciatic nerve's function and histomorphology. Methods According to the rat model of sciatic nerve compression designed by Mackinnon, the thirty two Sprague Dawley male rats were made the model of sciatic nerve compression as compression group; sixteen rats had the same operation but the silitube was took out as blank control group ;another 8 rats without operation as normal group. Sciatic Function Index(SFI) was determined in both blank control group and compression group every week.At the 2nd,4th week,2nd,4th,6th,8 th week and 2nd week,motor nerve conduction velocity(MNCV) was detected in blank control group,compression group and normal group respectively, with the sciatic nerve(for compression group, the compressed nerve)taken out to perform histomorphology analysis.Results\ At the 4th week, SFI and MNCV of blank control group had no significant difference to that of normal group( P >0 05),but that of compression group had significant difference to that of blank control group and normal group( P <0 05).Myelin sheath was first affected after compression.The changes of its morphology appeared,then follwed by changes of the thickness,at last the axon was affected. Demylination appeared at the 4th week. Conclusions\ The rat sciatic nerve compression model designed by Mackinnon had the strain injury to the nerve.And the compressed nerve showed histological changes of myelin sheath and axon.