Abstract: Based on the measurement of latent evapotranspiration (LE) and sensible heat flux by open-path eddy covariance (OPEC) system, the authors analyzed diurnal and seasonal variations of forest evapotranspiration in the broadleaved-Korean pine forests in Changbai Mountain in 2003. The results showed that the energy balance closure was 86.5%. This suggested that the latent heat flux and sensible heat flux measured by OPEC system at the forest site were reasonable according to the internationally reported energy closure range (60%-90%). The LE gaps were filled by multiple polynomial regressions on the net radiation (R_n) and air temperature (T_a). Forest evapotranspiration was higher in the daytime than that at night, with the highest value occurring at noon. The maximum of monthly forest evapotranspiration appeared in July and August, and the minimum appeared in winter months. The ratio of evapotranspiration to net radiation in the growing season was evidently higher than that in the non-growing season. The annual evapotranspiration amounted to 1 126.99 MJ/m² (450.8 mm of rainfall), accounting for 83.7% of the annual rainfall (538.4 mm).