In recent IAQ studies, there is much concern about air quality in school buildings. This paper focuses on indoor air quality in classrooms of two primary schools in Beijing, China. Ventilation rates, CO2 and PM2.5 are measured and analyzed. The results show that ventilation rates in naturally ventilated classrooms with windows and doors closed are less than 1 l/s/p, and CO2 concentration during class hours can increase over 2500ppm within a class. And also, closing windows and doors can only partly prevent outdoor PM2.5 from entering into indoor air, and indoor PM2.5 concentrations are 60∼70% of outdoor. To insure good indoor air quality in primary schools, mechanical ventilation devices with air purification function should be introduced in.