Evapotranspiration (ET) is an important component of water cycle, but its measurement in high altitude mountainous region is quite difficult, inducing the insufficient understanding on the actual ET in high altitude mountainous region and the effects of ET on this region' s water cycle. In this paper, two small type weighing mini-lysimeters were applied to measure the daily ET in a piece of grassland in a high altitude mountainous region of the Heihe River basin from July 1st, 2009 to June 30th, 2010. Based on the measured data, the methods of FAO-56 Penman-Monteith (F-P-M), Priestley-Taylor (P-T), and Hargreaves-Samani (H-S) were employed to estimate the ET to analyze the applicability of the three methods for the mountainous region, and the pan coefficient at the measurement spots was discussed. During the measurement period, the total annual ET at the measurement spots was 439.9 mm, accounting for 96.5% of the precipitation in the same period, and the ET showed an obvious seasonal distribution, being 389. 3 mm in May-October, accounting for 88. 5% of the annual value. All the three methods could be well applied to estimate the summer ET but not the winter ET, and their applicability followed the sequence of P-T > F-P-M > H-S. At the measurement spots, the daily pan coefficient in summer was 0.7-0. 8, while that in winter was quite variable.