Chlorine is an essential micronutrient and all crops require chlorine in small quantities. However, it is often associated with salinity damage and toxicity. Plants take up chlorine as Cl- ion from the soil solution. It plays some important roles in plants, including photosynthesis, osmotic adjustment, and transport of ions and nutrients throughout the plant. Also, there is even some research that shows chlorine can lessen the effects of fungal plant disease. In many cases, more than a sufficient amount of chlorine is supplied from the atmosphere and precipitation. Unlike other micronutrient elements, chlorine does not become toxic when accumulated in the plant, but symptoms of toxicity on the plant are related to the osmotic effects of salinity In this study, effect of irrigation water contains chlorine on population growth of Aphis gossypii Glover (Hem: Aphididae), Tuta absoluta (Lep: Gelechiidae) and Tetranychus urticae Koch (Acari: Tetranychidae) measured under laboratory condition. These pests were chosen because of their different feeding method. The granule of hypochlorite calcium Ca(Cl)2 70% were used in experiments. To prepare various concentration, amounts of 0.005, 0.01 and 0.05 grams of chlorine granules dissolve in 10 liters of distilled water. Concentrations of chlorine in distilled water reach 500, 1000 and 5000 ppm, respectively. Various concentrations of chlorine used in irrigation water of treatment plants. Distilled water used for control plants irrigation. All plants and pests used in experiments were synchronized aged. The results indicated that the effects of treatment of chlorine on population growth of T. urticae were significant in all concentrations, and T. urticae on treated plants had lower population growth in comparison to controls. Treatment of cucumber with chlorine had a significant effect on population growth of A. gossypii and aphids on plant treated with 1000 and 5000 ppm of chlorine had had lower population growth in comparison to controls. There was no significant difference between aphid population growth on 500 ppm chlorine and control. Number and size of galleries caused by larvae considered as damage index of Tuta absoluta. Number and size of galleries were significantly higher on control plants. Can be suggested that chlorine in irrigation water correlated with the reduced population growth of pests in treated plants. More biochemical and molecular studies suggested to find out the mechanism of this action.