During the summer season of 2002, emissions of volatile organic compounds were firstly measured by a static enclosure technique at a grassland site in the Inner Mongolia grassland. The parameters including solar radiation, air temperature and relative humidity were also measured in the same time period. The results showed that isoprene was the main component of volatile organic compounds emitted from the grassland. Isoprene exhibited an evident diurnal, daily and seasonal variation. Solar visible radiation, air temperature and water vapor content were the main factors to affect isoprene emission, and solar visible radiation was the most important factor to control the isoprene emission. Base on the correlation analysis, when considering the affecting factors of isoprene emission, it is not only to consider solar visible radiation, temperature, but also the water vapor. It is impossible to avoid the changes of solar radiation, temperature and humidity between inside and outside the chamber by using the enclosure technique. So, it is necessary to take these effects into the considerations and modify isoprene emission. In summer 2002, the maximum emission flux (C) of isoprene was 1649.3 micrograms/(m2.h), and monthly averaged emission flux (C) in June, August and September were 886.6, 707.0 and 427.2 micrograms/(m2.h), respectively.