The Ertan hydropower station at the Yalong River of Southwest Sichuan Province. It is one of the biggest installed capacity hydropower stations in China. The main stream reservoir length reaches 145km and the branch Ganyu River reaches 40km under normal storage level. The reservoir surface covers 101km-area, and has 5.8 billonm3 capacity. It began water storage since May 1998. The first generator was in operation in August 1998. The project was finished at 2000. The Ertan reservoir is a typical riverway style one. In order to understand the thermal effect, Dissolved Oxygen (DO) distribution and outlet flow temperature under reservoir storage condition, three field measurements were carried out in January, April, and July of 2000, respectively. The measured results are analyzed in the paper. The results will provide reserchers a valuable data set for calibration of mathematical model and realistic understanding for stratified flow issue and DO profiles in a large scale deep reservoir.