Armillaria root rot can cause mortality of pistachio trees and damage to orchards. Control of this disease using fungicides is very costly and in most cases unsuccessful. Therefore, the role of biocontrol agents especially Trichoderm species in controlling the disease is very important. In present study, the inhibitory effect of 8 isolates of Trichoderma harzianum and one isolate of T. crassum and T. asperellum isolates collected from pistachio orchards were investigated on Armillaria mellea growth. In dual culture test, A. mellea was cultured in 8 cm petri dishes containing potato-dextrose agar (PDA) and Trichoderma isolates at 2 cm spacing on both sides. In the test of volatile compounds, A. mellea and Trichoderma isolates were inoculated in center of petri dishes separately and the lid of each dish was replaced by a bottom containing PDA inoculated with a test fungus. The two dishes were taped together with parafilm, compeltely. The Petri dishes was kept so that petri containing Trichoderma was placed below. After 5 and 10 days, the diameter of A. mellea colonies was measured using a ruler and after comparing with the control, the inhibitory percent of the Trichoderma isolates was calculated. The results showed that different isolates of T. harzianum had a different inhibitory effect on the pathogen growth in dual culture and volatile compounds tests. In dual culture test, Th 825 (with 37%) and Th 8219 (with 29%) showed the highest and lowest inhibitory effect. Tc 8222 and Ta 8378 with 32 and 31% showed lower inhibitory effect compared with Th825. In volatile compounds test, Ta 8378 and Th8219 isolates had the highest inhibitory effect with 28.5 and 28.1%, and Tc 8222 with 13% had the least inhibitory effect on Armillaria growth. The inhibitory effect of the other isolates of T. harzianum was intermediate of these isolates. In general, the results of present study showed that the mechanism of inhibitory effect of different isolates of Trichoderma on A. mellea growth are varied and also there was a significant difference between the isolates belonging to one species in terms of effect on A. mellea growth. It seems that with supplementary studies and greenhouse tests, effective isolates of Trichoderma can be selected and used to control Armillaria root rot in pistachio orchards.