Oral toxicity of thymol, α-pinene, diallyl disulfide and trans-anethole, as well as their binary combinations, was studied on the fourth larval instars of Tribolium castaneum Herbst. Serial dilutions of chemicals were prepared from the stock solution to obtain the concentrations between 2.5-200 µl/ml prior mixing with 500 mg of the diet. LC50 values were determined as 6.79, 12.85, 8.52 and 1.03 µl/ml, for thymol, α-pinene, trans-anethole and diallyl disulfide after 24 hours and 3.74, 8.39, 6.48 and 0.68 µl/ml after 48 hours of exposure, respectively. LC50 values confirmed that diallyl disulfide and α-pinene were the highest and lowest toxic chemicals. The results show that combined treatment of thymol synergized α-pinene activity at 24 and 48 hours post treatments. In addition,“diallyl disulfide and thymol”,“thymol and trans-anethole” had additive effects on T. castaneum larvae. Our results suggest that the combination of thymol and α-pinene compounds were significantly more effective than each compounds alone on T. castaneum larvae.