Two-spotted spider mite, Teranychus urticae (Acari: Tetranychide), is one of the most important agricultural pests throughout the world. The high reproductive potential, short life cycle, combined with frequent acaricide applications has resulted in resistance development to a wide range of acaricides. Resistance to acaricides has an important role in inefficiency of chemical control of this pest. In the present study, the susceptibility of two populations of two-spotted spider mite collected from Karaj (KrS) and Mahallat (MhR) to spirodiclofen was investigated. The bioassay was conducted using a leaf-dip method on same-age protonymphs. Bioassay results showed that there was a significant difference between LC₅₀ values of KrS and MhR populations. The resistance ratio was obtained as 22.19. The synergistic effects of triphenyl phosphate (TPP), piperonyl butoxide (PBO) and diethyl maleate (DEM) were assessed using residual contact vial (RCV) bioassay method. Results of the synergistic studies showed significant differences between LC₅₀ values of spirodiclofen with spirodiclofen + PBO and spirodiclofen + TPP in MhR population. The most synergistic effect in MhR population was related to PBO. The results of enzyme assays revealed that the most ratio activity in MhR population to KrS population was related to cytochrome P450 monoxygenase (3.02) and the lowest ratio activity was related to glutathione S-transferase (GST) (1.40). These results confirmed that esterase and cytochrome P450 monoxygenase are probably involved in resistance of T. urticae to spirodiclofen.