Abstract: Mensurational and spatio-temporal changes of negative air ion concentrations in Beijing were studied between 1999 and 2002. The results showed that negative air ion concentrations increased gradually and ion polarity ratio decreased gradually from city center to suburb. The negative air ion concentrations in forested areas, were obviously higher than those in non-forested areas; their annual average values in coniferous forests were higher than those in broad-leaved forests but the opposite was true in spring and summer. The negative air ion concentrations obviously increased in the areas where moving water, such as waterfall and rivulet, exist. Indoor negative air ion concentrations were lower than outdoor ones but potted green plants could improve it. Over the span of a whole day it was higher during day time than during night time. Over a year its maximum appeared in summer and the minimum in winter.