Abstract: A technology of CO2 supercritical fluid extraction was adopted to remove the lignin in straw and bamboo. The content of Klason lignin and holocellulose was evaluated through orthogonal experiments. Extraction liquid composition was analyzed by GC/MS. The relationship between extraction liquid composition and experimental conditions was discussed. The ultrastructure of lignin distribution before and after extraction was analyzed by transmission electron microscopy(TEM) and energy dispersive X-ray analysis(EDXA). Results showed that temperature and time were major factors affecting delignification of straw and bamboo. The higher the temperature and the longer the time, the higher the delignification and the lower the holocellulose content. The extraction liquid consisted of five-membered rings, alkoxyl groups and aliphatic acids from cellulose and lignin degradation products(H-, G- and S-type structure units). TEM-EDXA analysis showed that after CO2 supercritical fluid extraction, cell wall delamination of straw was unobvious and the lignin of middle lamella and CCML in bamboo cell wall was removed at 200 ℃ and 16 MPa.