The aim of this research was to determine chemical composition and insecticidal activity of the essential oil of Illicium pachyphyllum fruits against two grain storage insects, Sitophilus zeamais and Tribolium castaneum, and to isolate any insecticidal constituents from the essential oil. The essential oil of I. pachyphyllum fruits was obtained by hydrodistillation and analyzed by GC-MS. A total of 36 components of the essential oil were identified, with the principal compounds in the essential oil being trans-ρ-mentha-1(7),8-dien-2-ol (24.56%), D-limonene (9.79%), caryophyllene oxide (9.32%), and cis-carveol (5.26%) followed by β-caryophyllene (4.63%) and bornyl acetate. Based on bioactivity-guided fractionation, the three active constituents were isolated and identified as trans-ρ-mentha-1(7),8-dien-2-ol, D-limonene and caryophyllene oxide. The essential oil of I. pachyphyllum fruits exhibited contact toxicity against S. zeamais and T. castaneum adults, with LD(50) values of 17.33 μg/adult and 28.94 μg/adult, respectively. trans-p-Mentha-1(7),8-dien-2-ol (LD(50) = 8.66 μg/adult and 13.66 μg/adult, respectively) exhibited stronger acute toxicity against S. zeamais and T. castaneum adults than either caryophyllene oxide (LD(50) = 34.09 μg/adult and 45.56 μg/adult) and D-limonene (LD(50) = 29.86 μg/adult and 20.14 μg/adult). The essential oil of I. pachyphyllum possessed fumigant toxicity against S. zeamais and T. castaneum adults with LC(50) values of 11.49 mg/L and 15.08 mg/L, respectively. trans-p-Mentha-1(7),8-dien-2-ol exhibited stronger fumigant toxicity against S. zeamais and T. castaneum adults, respectively, with LC(50) values of 6.01 mg/L and 8.14 mg/L, than caryophyllene oxide (LC(50) = 17.02 mg/L and 15.98 mg/L) and D-limonene (LC(50) = 33.71 mg/L and 21.24 mg/L). The results indicate that the essential oil of I. pachyphyllum fruits and its constituent compounds have potential for development into natural insecticides or fumigants for the control of insects in stored grains.