Verticillium wilt disease is one of the most important diseases of pistachio trees, which can cause mortality of pistachio trees in infected gardens. Chemical control of this disease is very difficult and the use of resistant cultivars is the only effective control method of the disease. Potassium silicate is one of the compounds that can help increase resistance of plants to pathogens by increasing mechanical barrier and strengthening of cell wall and also activating biochemical defense systems of the plant and increasing host resistance. In this research, the effect of 5 concentrations of 0, 10, 40, 100 and 200 mM potassium silicate was evaluated on verticillium wilt disease of pistachio seedlings (Ahmad Aghaei cultivar) under greenhouse conditions. Two month old pistachio seedlings were irrigated with potassium silicate concentrations after neutralization using 1 M solution of nitric acid. One month after application of potassium silicate treatments, pistachio seedlings with the roots and the surrounding soil were transferred to larger pots. At the bottom of these pots, a 3 cm layer of soil containing at least 40 microsclerotia of Verticillium dahliae was poured. Inoculation with V. dahliae alone, decreased shoot and root dry weight significantly compared to control. With increasing concentration of potassium silicate, the dry weight of shoot and root increased, so that the highest shoot and root dry weight (8.93 and 4.84 g per pot, respectively) was observed at 200 mM concentration of potassium silicate, which had significant difference with the control. In inoculation by V. dahlia alone treatment, the percentage colonization of stems was about 72% but in interaction of pathogen and 200 mM potassium silicate, stem colonization decreased to 28% and showed significant difference at 1% level. The use of potassium silicate increased the activity of catalase, peroxidase and phenylalanine ammonium lyase (PAL) enzymes in pistachio seedlings, so the highest activity of these enzymes (10, 39 and 3.5 units per gram of fresh weight of root, respectively) was observed at 200 mM of potassium silicate and had significant difference with control. The results of this study showed that the use of potassium silicate can decrease the percentage colonization of V. dahliae and damage of the disease in Ahmad Aghaei cultivar by improving shoot and root dry weight and increasing the activity of catalase, peroxidase and PAL enzymes.