In the present study, seventy-two 30-week-old laying hens (Brown Hisex) were separated into two groups (control and treatment) and fed with a standard layer diet. In the treatment group the amount of effective microorganism (EM) added to the drinking water is equal to1000 ppm EM dose. Throughout the 8-week study period, 16 h thermoneutral (20-22 degrees C) and 8 h hot (35-37 degrees C) environmental temperature regime was applied daily. The results indicated that EM supplementation affected performance and some egg quality characteristics of layers exposed high environmental temperature. Feed intake and conversion ratio, yolk index, albumen index, yolk colour b, Haugh unit were decreased by supplemental EM. Serum glucose, cholesterol, triglycerides, calcium, total oxidant-antioxidant concentration were not affected by EM. It is concluded that EM supplementation in laying hens could have potential to improve feed intake and feed conversion ratio under high environmental temperature. However, supplemental EM with drinking water decreased some egg quality parameters.