Stability of a two-wheeled inverted pendulum vehicle was evaluated by using rider-vehicle modeling, electromyography of leg's muscle of rider, and subjective evaluation. A rider and the vehicle are synthetically modeled as a series-type double inverted pendulum. Utilizing auto-regressive exogenous (ARX) model method, correlative relations were found between control gains and stability of the vehicle that achieves better ride comfort. The experimental result shows that the higher the control gains, the smaller the activities of leg's muscles. However, according to subjective evaluation of ride comfort, higher gains did not always achieve better ride comfort. Moreover, the rider-vehicle model has no unstable poles, even when the vehicle model has an unstable pole. The poles of the rider-vehicle model had tendency to have bigger negative real numbers, which suggests stability of the vehicle can be evaluated by the positions of the poles of the rider-vehicle model. Through identification of the rider-vehicle model, the control gains of the rider's posture were calculated. It was found that the control gain against the rider's posture is dominant, and the higher the control gains of vehicle, the smaller the gain against the rider's posture. The result shows stability of the vehicle can be evaluated by the control gains against the rider's posture.