Phytophthora root root is one of the most important diseases of pistachio trees caused by various species of Phytophthora. Silicate are salts that can inhibit the growth of several fungi. In this research, the effect of concentrations of 0, 2, 5, 15, 40, 100, 200, 400 mM potassium silicate and concentrations of 0, 1, 5, 15, 25, 60, 100 and 200 mM sodium silicate on mycelium growth of Phytophthora drechsleri (Pd), P. citrophthora (Pc) and P. parsiana (Pp) were studied. Different concentrations of silicate salts were mixed with corn meal-agar (CMA) and poured into 8 cm petri dish. At the center of each petri, a 6 mm disc was laid out from the margin of the young colonies of Phytophthora species, and the colonies diameter was measured at intervals of 3, 5, 7 and 12 days. The results showed that the reaction of three species of Phytophthora in different concentrations of potassium and sodium silicate differed from each other. At different concentrations of potassium silicate, 2,5,15 mM concentration increased mycelial growth of Pd compared to control and 200 and 400 mM concentarion inhibited mycelial growth completely. Pc mycelial growth increased to 5 mM and inhibited at 200 mM while mycelial growth of Pp decreased in all potassium silicate concentrations compared to control and inhibited at 100, 200 and 400 mM concentration completely. At the concentrations of sodium silicate, mycelial growth of Pd increased at 1 mM concentration (about 4%) compared to the control and stopped in 200 mM. Mycelial growth of Pc and Pp in all concentration of sodium silicate decreased in comparison with the control and inhibited completely (100%) at 100mM concentration.