Abstract: Complex internal components and operating conditions result to specific features of the three-dimensional flow and heat transfer phenomena in the upper plenum and the hot leg of VVER reactor. Thermal stratification leads to the temperature difference of various thermal resistances located in the same cross section of the hot leg. For the purpose of studying detailed thermal-hydraulic characteristics in VVER-1000 reactor, the real model of the upper plenum and the hot leg was built up by the three-dimensional CFD method based on reasonable simplification. From the numerical simulation, flow and temperature distribution of the whole computational domain was obtained, and concerned local detailed flow and heat transfer parameters were captured. In addition, analysis of factors affecting the thermal stratification in the hot leg was conducted. Results indicate that the maximum temperature difference reduces and insufficient coolant mixing occurs in the upper plenum. The largest temperature difference of 13 ℃ in the thermal stratification process and two significant reverse rotating flow paths exist in the initial cross section of the hot leg. The coolant mixing is due to radial and tangential velocity, and fluid temperature trends to be uniform in the downstream position of the hot leg. The coolant temperature and flow rate in core barrel holes with different elevations have a significant impact on the hot leg thermal stratification.