Root-knot nematodes (Meloidogyne spp.) are one of the most important plant parasitic nematodes in the world and cause serious damage to agricultural crops. In this study, cucumber seeds (cv. Super Dominus) were sown in 2 kg plastic pots and at four leaf stage, every each one of cucumber plants simultaneously were treated with a suspension of 107 spores/ml of one isolate of Trichoderma harzianum (Th42) as soil-drench (20 ml per pot) and were sprayed with one ml of salicylic acid (SA) at the rate of 3 and 5 mM. Twenty four hours after treatments, seedlings were inoculated with 1500 eggs and J2 of M. javanica per kg of soil. Plant growth and nematode population indices were assessed, 45 days after inoculation. In another experiment, time-dependent changes in activity of some antioxidant enzymes in cucumber leaves were investigated. Four leaf stage seedlings of cucumber simultaneously were treated with 107 spores/ml of Th42 as soil-drench (20 ml per pot) and sprayed with 3 mM SA (1 ml per seedling). One day after treatments, seedlings were inoculated with 1000 J2 of M. javanica per kg of soil. Activity of catalase (CAT), peroxidase (POX) and superoxide dismutase (SOD) in leaves of cucumber were investigated one day before nematode inoculation (Time 1), simultaneously with nematode inoculation (Time 2), one day (Time 3), two days (Time 4) and three days (Time 5) after nematode inoculation. Both experiments were set as factorial with completely randomized design with four and three replications for first and second experiments, respectively. The results of the first experiment showed that the combination of Th42 and 3 mM SA was the most effective treatment that reduced the number of eggs, galls, egg masses and the reproductive factor of nematode by 60.1, 41.3, 55.2 and 59.3%, respectively, as compared to control treatment, and 34, 27.9, 39.3 and 34.1% as compared to those plants that treated only by Th42, but did not show significant differences in growth indices of cucumber in different treatments. The results of the second experiment showed that the highest activity of CAT enzymes was observed at time points of 2 and 3, and SOD and POX at time points of 4 and 5. Considering the successful control of root-knot nematodes in vitro and in vivo, as well as the induction of plant defenses by T. harzianum, this fungus can be used as an effective biocontrol agent. Because the induction of defense activity by SA, it can be apply in root-knot nematodes management program. Combination of SA with T. harzianum was more effective than either agent alone.