In order to increase the reliability of the oil lubricated journal bearing, the thermal characteristics of the plain journal bearing are investigated and optimized. In this paper, the steady state thermo-hydrodynamic analysis of the plain journal bearing with centered circumferential groove is studied by numerical simulation and experiments. The diameter of the journal bearing is 190mm, and the load of the bearing is 6150N, which is operated with the rotating speed of 5560 r/min. The flow characteristics including the temperature distribution in the clearance of the bearing are simulated by ANSYS CFX, where the Walther viscosity temperature relation and the cavitation effect are considered. The test rig for measuring the pressure and temperature distributing in established, which is driven by electrical motor and certain bearing load can be exerted on it. In the experiments, the temperature distribution of the bearing is measured by 10 temperature sensors, which is arranged on three cross sections along the axial direction. The temperature of the lubricating oil is obtained by the sensors on the shell of the bearing, which is installed in the Babbitt metal. In order to obtain the circumferential distribution of the temperature, the sensors locate at different circumferential angles. The influence of geometrical parameters on the temperature distribution is studied, including the bearing clearance, the bearing length and the groove width by numerical simulation and experiment. The bearing clearance ratios of 1.8‰ and 2.5‰ are compared, and as the increment of the bearing clearance ratio, the temperature of the bearing decreased, which is benefit from the flow mass into the bearing increases. For the bearing length increasing from 90mm to 94mm, the maximum temperature decreases about 3K. For the groove width varying from 20mm to 19mm and 18mm, the temperature of the bearing decreases. The comparison of the numerical results and experiments are presented, and they show similar tendency for the temperature distribution, but the difference of temperature values exists. Based on the results from variation of bearing clearance ratio, bearing length and groove width, the optimized parameters of the bearing are proposed by determining the groove width with 18mm, bearing length with 94mm and the bearing clearance ratio with 2.1‰. The thermo-hydrodynamic analysis from the CFD and the experiments for the optimized bearing are carried out. The results indicate that the thermal performance of the bearing is improved. Compared with original design of the bearing, the maximum temperature is reduced by approximately 5K for optimized design of the bearing.