Abstract: Scolothrips longicornis Priesner is a native predator insect in Iran and it is an active biological control agent in bean, cucumber and eggplant fields. The aim of this study was to determine functional response and mutual interference of the predatory thrips, S. longicornis under laboratory conditions (25 °C, RH 65% and a photoperiod of 16 h light: 8 h dark) fed on Tetranychus urticae Koch on cucumber. The functional response was measured by offering eggs and nymphs of T. urticae at seven densities (2, 4, 8, 16, 32, 64 and 128) to an adult female of thrips separately and the mutual interference was calculated by five densities (1, 2, 4, 8 and 16) of adult female predators on 250 nymphs of T. urticae. Both experiments were done during 24-h period. The values of the attack rate (a) and handling time (Th) were estimated using the Royama-Rogers’ model. The logistic regression showed a type II response of the predator to different densities of eggs and nymphs of T. urticae. Both parameters (a, Th) were very similar between eggs (0.1107 and 2.4038 h, respectively) and nymphs (0.1034 and 2.4691 h, respectively). The per capita searching efficiency decreased significantly from 0.040 to 0.024 with increasing predator density from 1 to 16 females. Also the per capita predation decreased significantly from 9.800 to 4.983 with increasing predator density from 1 to16. The results revealed the high predation potential of S. longicornis, which it can be used as a sufficient biological control agent on T. urticae.