The black soil area in China is typically characterized by freeze-thaw alternation, however, little is known about impacts of the freeze-thaw cycles on N availability in black soil. An indoor bacth experiment was conducted to investigate effects of freeze-thaw cycles different in temperature range (-25—5 ℃ and -10—5 ℃) on net nitrogen transformation and soil enzyme activity in black soil. Soil ammonium nitrogen and nitrate nitrogen content was determined with the 2 mol L-1 (1:5 soil liquid ratio) KCl extraction method, and analyzed with the automatic chemical analyzer, Smartchem 200. Soil urease activity measured with the indophenol blue colorimetric method and soil invertase activity determined with the 3,5-dinitrosalicylic acid assay method. Results show that the contents of ammonium nitrogen and nitrate nitrogen increased gradually with frequency of the freezing and thawing alternation and varied with intensity of the cycle. In terms content of ammonium nitrogen, the following order was observed: cycles moderate in intensity (-10—5 ℃) > cycles high in intensity (-25—5 ℃) > control (5 ℃), except for the first round of freeze-thaw cycle moderate in intensity. However , freezing-thawing cycles did not have much influence on soil nitrate nitrogen. On the whole, freeze-thaw cycles reduced soil nitrogen mineralization and nitrification rates,, and the effect was obviously affected by the intensity of freezing and thawing, but at the end of the experiment, the cycles displayed a tendency of promoting nitrification. As affected by low temperature and the alternation, the activities of urease and invertase in the soil under the cycles moderate in intensity were higher than that in the soil under the cycles high in intensity (except for the first cycle), however, both were lower than control. So the effect of soil freezing and thawing on black soils has an important influence on nitrogen transformation and soil enzyme activity. Urease activity and soil nitrogen supply capability are significantly correlated. Besides, invertase activity is affected by soil geochemical cycles through carbon source supply and temperature.