This paper describes to evaluate the performance of ventilation as a change of CO2 concentration with ventilation rate under 35 occupants in a school classroom. Variations of the CO2 concentration as a function of time are measured at center point of classroom with air ventilation rates by the CO2 gas monitor. For 800 m3/h of ventilation rate in the classroom, the average CO2 concentration is 913 ppm. The ventilation rate is sufficient to meet acceptable indoor air quality (≤ 1,000 ppm of CO2, KS school standard) in the classroom of 35 students. Keywords : Indoor air quality, Ventilation, Carbon dioxide