Four broiler houses located on the same farm belonged to Emivest Vietnam Co., Ltd in Binh Phuoc province were selected to evaluate the effect of indoor environmental factors including temperature (oC), relative humidity (%), air velocity (m/s), concentration of O2 (%), NH3, H2S, CO and CH4 (ppm) and the presence of Escherichia coli and Eimeria spp. in houses on growth rate, feed intake and feed conversion ratio of one to 42 day old chicks. All birds were kept in tunnel-ventilated houses and raised on deep litter floor, which was divided into 4 equal positions of 360 m2. The measurement showed that there was no harmful gas detected, O2 concentration was maintained at normal level (20.9 vol%). Number of fecal E.coli was 14*106 CFU/g with no presence of Eimeria spp. The final weight of birds reared near vent fans was higher to that of birds in the exhaust fans (3,026 g vs 2871 g, p