In order to study the diurnal and seasonal variations of CO2 fluxes and water use efficiency(WUE),the open-path eddy covariance systems were employed to continuously measure carbon and energy fluxes of the Alpine meadow during the three growing seasons(from May to October) of 2003-2005,respectively.The results showed that:(1)The maximum global solar radiations were 1 563W/m₂ and 1 640W/m₂ in 2004 and 2005(mean value of 30 minutes),respectively,greater than the solar constant;while the maximum net radiations were 896W/m₂ and 925W/m₂,and the maximum latent heat fluxes densities were 592 W/m₂ and 597W/m₂,respectively.As for the six selected days from May to October in 2004,the ratios of latent heat flux to net radiation were 0.355%,0.916%,0.738%,0.818%,0.609% and 0.456%,respectively.(2)The latent heat had an obvious diurnal variation.During the daytime of growing season,the latent heat fluxes density increased with radiation,and declined after reaching its maximum at 15:00p.m;CO2 flux reached its maximum at about 10:30a.m.and WUE presented a falling trend from morning to afternoon.The maximum CO2 fluxes and WUE(mean value of 30 minutes) were similar to 2004 and 2005,with values being 0.3 mg CO2·m-2·s-1 and 8g CO2/kg H2O,respectively.(3)The CO2 fluxes and WUE(mean value of the daytime) varied obviously at the seasonal and yearly scales.The carbon fluxes(mean value of the daytime) represented as a carbon sink from early June in 2004,but from late June in 2005;WUE(mean value of the daytime) became positive also from early June in 2004,but from late June in 2005;in addition,the overall values of carbon fluxes and WUE(mean value of the daytime) of 2004 were greater than 2005.The maximum carbon fluxes were 0.15mg CO2·m-2·s-1 and 0.13mg CO2·m-2·s-1 in 2004 and 2005,respectively.And the maximum values of WUE(mean value of the daytime) were 3.2g CO2/kg H2O and 2.7g CO2/kg H2O,respectively.(4)The total net CO2 absorption of the growing season(from May to October) in 2004 and 2005 were 0.257kg CO2·m-2 and 0.153kg CO2·m-2,respectively.The water use efficiency throughout the growing season in 2004 and 2005 were 0.496g CO2/kg H2O and 0.365g CO2/kg H2O,respectively,being positively correlated with seasonal precipitation.