Residual potential is a very important performance index of organic photoconductor (OPC). At present, research shows that the purity of charge transport material will seriously influence residual potential of OPC. But in past research we found that some OPC used charge transport material with very high purity still has very high residual potential. With quantum calculation and x-ray diffraction we found that some materials are optical isomer and some have cis-trans isomerism. So in order to improve performance of OPC we should separate isomerism.