Based on climatological standard normals of evaporation and precipitation at an urban area, Nanjing(1960～1990), the variation of two climatic elements in urban forest in Nanjing were analyzed using daily data measured at two meteorological observing stations(an urban site and an open site of the ecological station) from 1988 to 2002. The characteristics of the air moisture were also described by the normals, annual range, month-to-month difference, amplitude of monthly variation and extremum etc. During the past 15 years,the normals of annual evaporation in the urban forest site were 19% less than that in the urban site,mean differences of seasons at two sites were a maximum(25%) in autumn and a minimum(16%) in spring. The normals of annual precipitation in the urban forest site was 4% more than that in the urban site. The mean difference of seasons at two sites was the most(10～) in winter. The result indicated that the urban forest plays an important role in maintaining air moisture near the surface atmospheric layer.