Abstract The large power mining dump truck usually has electric drive system for the harsh operating conditions of mining. The traction motor and multi-stage reducer are assembled in the limited space of the two rear wheels. The permanent magnet motor is often used as traction motor for its much advantage characteristic. However, the permanent magnet is particularly sensitive to the high temperatures since it can loose a part of magnetization when it is exposed to a high temperature. It is necessary to have better cooling system for improvement thermal performance of permanent magnet (PM) motor. The heat losses generated by permanent magnet motor in the off-road electric driving truck are cooled by the water to maintain the motor working effectively. This paper is focus on analyzing the thermal behavior of the PM motor numerically and experimentally. The 2-D transient temperature field of motor is investigated based on different losses of the permanent magnet motor which is determined by the magnetic field analysis. The water cooling housings with different channel layout are compared with the synthetic evaluation index which reflecting the comprehensive efficiency of cooling system. The 3-D CFD model of water cooling housing is developed to present the velocity and temperature field of the cooling water flowing in the circumferential channel. The temperatures of different part of PM motor with water-cooling housing are measured under different operating conditions. The experimental results show the validation and effectiveness of the simulation model in predicting thermal characteristics of the traction motor. 1. Introduction Off-road dump truck generally operates in the complicated condition, such as open-pit mine, construction site, and quarries etc. Most large size off-road dump trucks employ diesel/electric powertrains, using the diesel engine to drive an AC alternator or DC generator that sends electric power to electric traction motors at each rear wheel. Instead of mechanical powertrains with gearboxes and driveshafts, diesel electric trucks typically are easier to operate, cost less to purchase and generally outperform mechanical trucks, especially on steep grades. Permanent magnetic(PM) motors possess too much characteristics, such as: compact space, higher torque density, less weight, wider speed range, larger starting torque and short time overload capability, so it is usually used as traction motor for off-road dump truck. This higher power PM motor owns a great compactness for its limited space constraint. The most part of the absorbed electric power is converted into mechanical power and drives the motor into rotation. The residual loss of this energy is used to heating up the motor. However, the performance of PM motor is sensitive to the temperature of stator and rotor. And the PM traction motor, mounted in the wheel rim with the wheel reducer, will generate amount of heat which is difficult to distribute to the ambient. This phenomenon would accelerate the aging of the insulation material and limit the actual output torque of the motor. So, the cooling system should have reasonable design structure to enhance cooling capability of PM motor in the limited space. Many studies on the thermal performance of PM motor have been investigated through the experimental and numerical analysis. FEM has its advantage to solve the engineering problem with complex boundary. It is suitable to precisely describe the thermal distribution of motor, determine position and value of the hot point. Paper[ 1] analyzed the magnetic field to calculate the heat loss of PM motor, and the thermal field of PM was simulated by these heat losses. Paper[ 2] built 2-D model of squirrel induction motor to predict the thermal behavior of the motor at different working conditions. Paper[ 3] investigated the flow and thermal field of end-winding region of PM motor, and evaluated the influence of structu