Meloidogyne species are the most important parasitic nematodes infecting cucurbits plants. These nematodes cause 5% loss in crop production and provide favorable conditions for co-infection with other plant pathogens. Biological control is one of the best eco-friendly methods to manage plant diseases. Trichoderma harzianum Rifai is one of the most important biological control agents for controlling the plant pathogens, including plant parasitic nematodes. By competition with nematode and colonizing the root, as well as producing antibiotics and enzymes and stimulating plant defense mechanisms, this fungus inhibits the development and reproduction of plant parasitic nematodes. In order to find the effective isolate/s of this fungus for management of root knot nematode of cucumber, an experiment was conducted as completely randomized design with four replicates. Seeds of cucumber (cv. Super Dominus) were sown in pots with 500 g of sterilized soil mixture with equal proportions of farm soil, farm yard manure and sand. The pots were kept under controlled conditions of greenhouse and temperature was adjusted to 28 ± 4 ºC with 16:8 h light:dark photoperiod. Plants were regularly irrigated based on plant demand. At four leaf stage, the cucumber seedlings were treated with 20 ml of a suspension of 107 spores/ml of the isolates 36, 39, 42 and 43 of T. harzianum as soil-drench, per plant. After 24 hours, treated seedlings were inoculated by 2000 eggs and second stage juveniles (J2) of Meloidogyne javanica (Treub 1885) Chitwood 1949. Forty-five days after nematode inoculation, plant growth and nematode population indices were evaluated. The results showed that the length, fresh and dry weight of shoot and fresh weight of root in plants treated with different isolates of this fungus were not significantly different with control treatment. The isolate Th42 was the most effective isolate that significantly reduced the number of eggs, galls, egg masses and reproductive factor of nematode by 69.2, 81.8, 88.6 and 70%, respectively, as compared to control treatment. Due to the successful control of root-knot nematode in the greenhouse by this isolate of T. harzianum, it can be used as an effective bioagent for management of M. javanica in greenhouse cucumber