One of the antagonistic mechanisms of Trichoderma species against the pathogens is to stimulate the host defense reactions including an increase in the activity of hydrolytic enzymes. In this study, the effect of two isolates Trichoderma harzianum-136 and Trichoderma harzianum-8279 ( was evaluted on the activity of β-1,3 glucanase and cellulase enzymes and leaf chlorophyll content in two pistachio rootstocks, Sarakhs and Badami-Zarand, infected by Phytophthora drechsleri , the causal agent of Phytophthora root rot. The pistachio seedlings were inoculated with the pathogen after the their colonization with the Trichoderma isolates. When the disease symptoms appearedin the control plants, the activity of β-1,3 glucanase and cellulase enzymes and leaf chlorophyll content were measured in the treated plants. Inoculation of the seedlings withP. drechsleri reduced the chlorophyll content of leaves in Sarakhs (37%) and Badami-Zarand (17%) compared to the control seedlings. The hydrolytic enzyme reduction was only significant in Sarakhs cultivar. In contrast, leaf chlorophyll content of both pistachio rootstocks increased by Trichoderma colonization compared to the controls. This increase was only significant in T. harzianum-8279. In Sarakhs cultivar, the chlorophyll content of leaves increased only in the interaction of T. harzianum-8279 and P. drechsleri treatment compared to inoculation with P. drechsleri-alone.Inoculation with P. drechsleri significantly reduced the activity of β-1,3 glucanase (59%) and cellulose (60%) in Sarakhs, but in Badami-Zarand, the activity of both enzymes increased, which was due to higher tolerance of Sarakhs to P. drechsleri. Inoculation of T. harzianum-8279 increased the activity of β-1,3 glucanase (12 and 17.3 U/gram of feresh weight of root) and cellulase (9.9 and 12.7 U/gram of feresh weight of root) in Sarakhs and Badami-Zarand which showed significant difference with the control. The interaction of Trichoderma isolates with P. drechsleri on both pistachio rootstocks increased the activity of β-1,3 glucanase and cellulase enzymes compared to inoculation with P. drechsleri alone , which only significantly differed in T. harzianum-8279 isolate. It seems that the increase of hydrolytic enzyme activities by Trichoderma isoltes is one of the important mechanisms in increasing the tolerance of pistachio seedlings to Phytophthora root rot.