Abstract: Based on the in situ investigation data obtained during the period from June in 2001 to May in 2002 in Tianmuhu Lake,the paper analyzes the vertical and spatial distribution and yearly and daily variations of water temperature,and reveals the distribution and variation of thermocline and the relationship between water temperature and dissolved oxygen and nutrient.The results suggest that the hourly maximum temperature of surface water of Tianmuhu Lake occurs between 14:00 and 19:00 and the hourly minimum temperature of surface water occurs between 0:00 and 8:00.The yearly variation of water temperature is summer > spring,autumn > winter and with a maximum value in July or August and a minimum value in January or February.There exits obvious temperature difference between surface and deep layer water in spring,summer and autumn.Thermocline turns up from March to October,and strongthens from March to May,then weakens from September to October.The variation of water temperature has an obvious effect on the values of dissolved oxygen and the concentration of nutrient.When the thermocline is found in the lake,the value of dissolved oxygen on surface is less than that of the deep layer.However,when the thermocline disappears,the difference between surface and deep layer is minor.Moreover,the thermocline prevents the nutrient exchange between the surface and the deep layer water.