Abstract: Improved Sweet Orange(Gailiangcheng)was a good weet orange varity which was obtained from the sweet orange(Citrus sinensis var.liucheng)grafted on the stock of red tangerine(Citrus reticulata Blanco).This experiment has been carried out with polyacrylamide gelelectrophoresis.According to variant types of Improved Sweet Orangefruits,the perxidase isoenzymes of vesicles、flaved and endocarp of red pulp type、yellow pulp type and chimera of red and yellow pulp type were determinde.the central region of peroxidase isozyme pattern was analysed and compared with the Enzhi Gan(Citrus sinensis var.liucheng)and Hong jie(Citrus reticulata Blanco).It was further proved that the fruit of Improved Sweet Orange was a Chimera,and it was suspected that the histogen of Improved Sweet Orange trees on that farm was a periolinal chimera,in which L-Ⅰ、L-Ⅱderived from red tangerine and L-Ⅲfrom sweet orange.