As a very important part of urban green space system, green belt plays an irreplaceable role in the construction of green space and improvement of ecological environment. In order to provide scientific basis for the construction of urban green belts, the effects of the different width of urban green belts on the temperature and humidity were analyzed by the small-scale of quantitative measurement. In this paper, six-green belts with different width including 6 m, 16 m, 27 m, 34 m, 42 m, 80 m along the west fourth ring road of Beijing were investigated in July of 2009. The air temperature and relative humidity in six- green belts were measured once every two hours from 8: 00 to 18: 00 every day for continuous seven days. The results show: (1)The temperature and relative humidity benefit increases as the width of six-green belts rises. The green belt of 6 m width has a little effect on the decrease of temperature and unobvious effect on the increase of humidity; It has an comparatively obvious effect with the width of 16 27 m; an obvious effect with the width of 34 m; an extremely obvious and stable effect with the width of more than 40 m. Analysis of variance shows that the differences of air temperature and humidity between six-green belts with different width and control group reach significance level. Based on Duncan'S Post Hoc Tests (P = 0.05), it can be concluded that the critical width of urban green belt for the obvious effect on the decrease of temperature and the increase of humidity is around 34 m (green cover percentage must be about 80%), as the effect is better, and the optimum width is around 42 m (green cover percentage must be about 80%). (2)There is a certain effect of green belts on surrounding byway, but this effect dose not reach significant level because the temperature and relative humidity of byway is affected by the surrounding environments. (3)Regression equation Y (humidity) = 184.721-4.038x (temperature) is built based on regression analysis between the temperature and humidity in six different width green belts, which indicates that the relative humidity decreased by 4.0% with every 1.0 ℃ rise.