Aphis fabae is one of the most damaging pests that attacks more than 200 plant species and carry more than 50 types of plant diseases. Considering the indiscriminate use of chemical toxins against this pest, particularly Chlorpyrifos, determination of a proper dose of this toxin for farmers' uses seems necessary, so that we may be able to minimize the remaining amount of this toxin in the environment. Chlorpyrifos is a part of the group of OP insecticides exposure, gastrointestinal effect and a little fumigant property. It plays its role by affecting the normal function of the nervous system. In this research a completely randomized scheme with seven treatments and five replications has been used in order to find the proper concentration of the toxin Chlorpyrifos to control the aphis fabae in laboratory and greenhouse. In both conditions (laboratory and greenhouse) after 96 hours, a significant difference was observed on the n probability level of one percent between treatments. The results showed that the 0.07ppm concentration is the most effective concentration of the Chlorpyrifos toxin in laboratory and greenhouse conditions, and also with the help of Polo-PC software, Chlorpyrifos' LC50 in laboratory and green house conditions for aphis fabae was respectively obtained 0.008 and 0.05ppm.