[Objectives]  To determine the effectiveness of a dispersible oil suspension of Lecanicillium lecanni for the control of Aphis gossypii. [Methods]  The toxicity of the suspension to A. gossypii was determined by the impregnation method and its effectiveness in a greenhouse by the five-point sampling method. The effect of temperature and humidity on its toxicity was determined by conducting trials under controlled temperature and humidity. [Results]  Two dispersible oil suspensions of L. lecanni (1and 2) were found to be the most toxic after 120 h with LC₅₀ values of 1.21×10⁵ cfu/mL and 2.08×10⁵ cfu/mL, respectively. There was a significant difference in the concentrations of the two formulations on the 14th day after application in the field. The best protection was provided by 4×10⁶ cfu/mL and 2×10⁶ cfu/mL of formulation 1, which had a lethality of 97.89% and 97.87%, respectively. There were also differences in the optimal temperatures for the two formulations. The optimal temperatures for Formulation 1were 25 ℃ and 28 ℃, at which the adjusted mortality rates on the 5th day of 96.34% and 94.09%, respectively. The optimum temperature for Formulation 2 was 28 ℃, at which the highest corrected mortality rate was 93.03%. There was no significant difference between the two formulations under the same relative humidity conditions; the optimum relative humidity was 67% of higher 5 days after treatment at which mortality was > 90%. Mortality for both treatments was around 70% at a relative humidity of about 30%. [Conclusion]  Dispersible oil suspensions of L. lecanni are highly toxic to A. gossypii and ambient temperature and humidity have little effect on their toxicity to this pest. Such dispersible oil suspensions therefore appear to be a useful fungal insecticide for controlling A. gossypii populations.