253, 249 and 230 water samples from wells in Chao Lake watershed were respectively obtained in winter(2009), spring(2010) and summer(2010), and analysed for nitrate-N to discover its spatial and seasonal patterns. Groundwater nitrate contamination in Chao Lake watershed is severe, which more than 20% of shallow wells sampled in winter, spring and summer, respectively, show groundwater nitrate-N exceed 10 mg/L, the WHO limit for drinking water. The groundwater nitrate-N concentration in the north region of Chao Lake watershed is higher than that in the south region. The nitrate-N concentration in the south region decreases from western hill to eastern plain while that in the north region is lower in north-east part which is the divide between Yangtze River and Huai River. Rank of nitrate-N concentration in different land uses is village > vegetable land > dryland > town > rice-rape (or wheat) cropping field > orchard > single cropping rice field > livestock farm. In general, the nitrate-N concentration highlights significantly seasonal pattern with that being almost the same in winter and spring, and higher than that in summer from the whole Chao Lake watershed. However, seasonal patterns of nitrate-N concentration are different in different land use types, which the pattern of winter > spring > summer is found in vegetable land, orchard and rice field, that of spring > winter > summer in dry land, town and livestock farm, and that of spring > summer > winter in village, possibly due to different time and amount of fertilizer application.