In the Nile delta of Egypt, small farmers raise their animals inside closed pens with bad ventilation without considering the health conditions. The objective of this work was to study the impact of 1) different ventilation rates on barn air condition to determine the optimum ventilation rate (VR) inside the barn. 2) poor and optimum VR on Egyptian buffalo calves’ welfare. This study was carried out at the research unit for animal behavior, Faculty of Agriculture, Menoufia University, Egypt. Ten Egyptian buffalo calves aged about 5-6 months and the average weight of 154 ± 17 kg, tied in closed pens with concrete floor were used. The calves were allocated in two similar groups, each 5 animals in a similar pen, one as control with limited natural ventilation and the other (treatment) supplied by mechanical ventilation. Four treatments A, B, C, and D (by using 2, 4, 6, and 8 fans, respectively) were applied to determine the optimum VR as an initial experiment (object. 1). The optimum VR was achieved by Treat. (D) in which the lowest temperature of 18.43 °C, the relative humidity of 59.33%, THI of 55.56, and CO2 concentration at 401.87 ppm. Calves’ performances (growth rate, CBC and behavior) and environmental variables (T, RH, and CO2 con.) were recorded for 60 days in control and Treat. (D) (object. 2). The results showed decreased values of T and RH inside the treatment pen than control (18.43 °C and 56.65 % vs. 23.49 °C and 62.86 %, respectively).