Objective To determine the synergists action of piperonyl butoxide (PBO) and S,S,S-tributyl phosphorotrithioate (DEF) on toxicity of carbamate insecticides against Blattella germanica in Tehran city. Methods In the current study, German cockroach strains were collected from several hospitals and dormitories in Tehran. At the beginning, different concentrations of bendiocarb and carbaryl (insecticides belong to carbamate group) were determined by surface contact on a susceptible strain. Then, the level of susceptibility and type of resistance mechanisms in the collected strains from contaminated sites to the aforementioned insecticides were studied by using PBO and DEF synergists with different insecticide ratios to synergist (1:0, 1:1, 1:2, 1:3). Results The DEF synergist along with bendiocarb and carbaryl completely eliminated the resistance in all strains but PBO did not completely eliminate the resistance in the strains of Mofid, Alvand, Valiasr hospitals and Shariati dormitory. Generally, the impact of DEF was observed in the removing resistance more than PBO. Conclusions In most of these strains, resistance to bendiocarb and carbaryl is completely eliminated by DEF, showing a very high role of estraze enzymes in resistance to bendiocarb and carbaryl. But in most strains PBO does not remove the resistance because other mechanisms, such as reduced cuticle penetration and insensitivity to the acetylcholine esterase enzyme, may be involved.