Previous experimental measurements showed that heat transfer performance of heaters submerged in a water pool is enhanced when ultrasonic vibration is applied to the pool. The increase rate in CHF values varied depending on the inclination angle of the heated surface with the maximum when horizontal and facing downward. Despite the obvious enhancement of heat transfer performance, it was not clear how ultrasonic vibration influenced heat transfer in a pool. In order to understand the mechanism, the experimental conditions inside the water pool with and without ultrasonic vibration were numerically analyzed. A commercial CFD package that has a dynamic mesh model, FLUENT V. 6.1, was used in the analysis. The results show that the standard deviation of temperature distribution in the pool is reduced when ultrasonic vibration is applied to the pool. The reduction rate of standard deviation of temperature is greatest when the heated surface faces downward and decreases as the inclination angle of heated surfaces increases. This trend coincides with the trend of CHF increase rate. Based on these results, it is concluded that an ultrasonic vibration enhances fluid mixing around the heater and thus provides heated surface with fresh water. This has an effect of lowering liquid temperature adjacent to heater surface, and in turn, leads to a heat transfer augmentation.