Recently, two-wheel electric powered wheelchair (EPW) attracts attention since their mobility is better than that of conventional four-wheel EPW. However, it is statically unstable due to underactuated structure. Then, stabilization control needs to be achieved whenever disturbances are added by road environment. In this paper, novel structure of two-wheel EPW is proposed. Controlled casters are attached at the body of two-wheel EPW and it can change two-wheel drive and four-wheel drive by position control of controlled casters. Furthermore, an assist control of step climbing motion in two-wheel EPW is proposed. Reaction Torque Observer (RTOB) and Repulsive Compliance Control (RCC) are used in order to generate the optimal pitch angle command for climbing the step. Furthermore, pitch angle command is transformed into the position command of controlled casters based on geometric equation. Then two-wheel EPW achieve step climbing motion easily by controlled casters. Experimental results are evaluated in terms of the comfort of human based on ISO 2631-1.