Downhill turning prevention control (DTPC) can help users of power wheelchairs (PWC) to maintain a straight track on cross slopes. DTPC detects the gravity-induced and unintended change in the travelling direction of PWC and offsets it by modulating the motor power. Whereas some PWC suppliers have commercialized the DTPC system, little attention has been paid on its effects on the users. In this study, we attempted to obtain quantitative evidences that show the effects of DTPC on joystick operation of a PWC. Seven subjects with cervical spinal cord injury drove a test PWC with DTPC on a cross slope. During the test drive, joystick operation and behavior of the PWC were monitored and recorded. The comparison of several evaluation measures from the trial records with and without DTPC clearly elucidated changes in joystick operation. First, the amount of joystick displacement in the left-right (LR) direction significantly decreased with DTPC. This is a direct evidence that DTPC replaced the manual offset operation by the subjects to prevent downhill turning. Second, the amount of joystick displacement variation in the LR direction also decreased with DTPC. This evaluation measure is correlated with stability of joystick operation. Therefore the result here indicates that DTPC enabled the subjects to operate the joystick more stably by reducing the burden of the offset operation. Lastly, we found the other side effect of DTPC on joystick operation, i.e., the slight increase of the amount of joystick displacement variation in the forward-backward direction. This result implies that the DTPC-induced decrease of the offset operation made it easier for the subjects to adjust the speed of the PWC more frequently and precisely. These results shown here demonstrated various positive effects of DTPC on joystick operation.