In this study, 20 isolates of Trichoderma were examined for their effectiveness as biological control agents of root knot nematode. The isolates were cultured in liquid culture medium containing colloidal chitin. According to results, isolates 125, 126 and 10 Bi of Trichoderma harzianum showed maximum chitinase activity compared to others. In greenhouse experiments, biological control potential of 6 isolates with maximum or minimum enzyme activity against the nematode was compared. Results showed that isolates with maximum rate of enzyme activity in laboratory experiments had better performance in greenhouse studies. T. harzianum 125 induced an increase in the activity of phenylalanine ammonia lyase and peroxidase in the roots of tomato plants.