An effective method was developed to convert the cellulose of delignified and non-delignified waste-cellulose fibers of paper and woodwork industry by the cellulolytic fungi Pleurotus sajorcaju. These cellulosic materials are strongly resistant to direct enzymatic hydrolysis. For this reason, the cellulose fibers were effectively pretreated with physical and chemical pretreatments. It was observed that evolution of cellulases activity depends on the type of pretreatment, the substrate utilized and the culture age. However, the highest ignocellu activity was recorded in acid pretreatment of delignified and non-delignified waste-cellulose fibers of paper and woodwork industry.