Abstract: In order to study the effect of the diameter of leachate drainage pipeline on operational characteristics of semi-aerobic landfills, three simulated semi-aerobic landfills with leachate recirculation were constructed in laboratory using different diameters (80, 50, and 25 mm) of leachate drainage pipelines, and the variations in temperature of the waste, quantity of the leachate, quality of the leachate, etc., during 344 days were analyzed. The results show that the diameter had an obvious influence on the operational characteristics. With the increase of the diameter, the quantity of leachate decreased, the pH of leachate rose quickly, and the leachate quality indexes such as the chemical oxygen demand, NH3-N concentration, and total nitrogen concentration improved rapidly. Under this experimental condition, the organic in the waste could be degraded completely when the diameter ratio of leachate pipeline to waste heap was no less than 1∶32. On the other hand, when the diameter ratio reached 1∶16, the organic could be degraded completely, and the nitrogen compounds could be reduced efficiently.