As a significant part of water balance, dew has attracted great interest and has been extensively studied. Dew is a crucial input for water balance and nutrients cycling of farmland ecosystem. Dew could play important role in spaying leaf fertilizers in paddy and drought-resisting and preventing the disease and pest in rain fed land. In this study, a series of parameters about dew characterization including Dew Days, Dew Frequency, Dew Intensity and Dewfall were defined comparing to parameters describing the precipitation. Based on these parameters dew condensation was measured in a farmland in Sanjiang Plain. In order to discuss the condensation law of dew, Dew Intensity was monitored in situ on stem and leaf of plant in paddy, soybean and corn fields during the growing season form early of June to late September in 2008 and 2009. The experiment was organized at the Sanjiang Mire Wetland Experimental Station (47°35′N, 133°31′E), Chinese Academy of Sciences, located in Tongjiang, Heilongjiang Province, Northeast China. Dew was collected by wood sticks made with poplar tree. The collector was daily weighted at sunset and sunrise with an electronic balance (accuracy is within 0.001 g). For each experiment, the collectors were set on the bottom (5 cm above the surface level) and canopy of the plant, respectively. The results indicated that dew occurred about 70 days in a year and Dew Days accounted about 70% non-rain nights in the farmland. The maximum Dew Intensity occurred in July and August. Statistical analyses were carried out to compare the Dew Intensity. Analysis of variance of Dew Intensity was performed using one-way analysis of variance (ANOVA) and the significance was set at P<0.05. The Dew Intensity in 2009 was significantly larger (P<005) than that in 2008 because the rainfall in 2009 was much more abundant than that in 2008. The Dew Intensity on paddy and soybean plants were almost equivalent, but the Dew Intensity on corn plant was significantly (P<0.05) lower than those on paddy and soybean plants. The vertical condensation laws of dew on various plants were similar. The Dew Intensity was maximum at the canopy, the second was at the height that was 50 cm over the canopy, and the minimum occurred at the ground level. Dew plays an important role in the water balance of farmland ecosystem. The factual Dewfall in unit field area was calculated by the factors of leaf area index (LAI), Dew Days and Dew Intensity. The annual Dewfall showed a distinct variety of the amount between paddy and rainfed land. The annual dewfall in rainfed land ranged from 10 mm to 15 mm. In the paddy Dewfall was about two to three times larger. The Dewfall increases rapidly when rainfed land reclaimed into paddy. LAI was the determining factor that affected the annual dewfall. Otherwise, the formula for calculating Dewfall offered a reliable way to analyze dewfall data in farmland ecosystem. Most importantly, dew parameters, which were defined for the first time, provide a more precise way to describe and study dew.