Power assist wheelchair robot is playing a key role in the field of medical welfare robotics. But, to a certain extend, there is a setback in development due to presence of caster wheels. Hence, this paper proposes a casterless wheelchair and its control on a straight line path. That is, Straight line driving of two wheel power assist wheelchair robot was proposed in this paper while keeping stability of wheelchair body itself. Balancing of wheelchair body is proposed to achieve by controlling the inverted pendulum mounted on the wheelchair. Control equations of the system are derived using Lagrange equation of motion. Two separate controllers are designed for two wheels. In addition to these two controller, there is another controller for pendulum. Body controller is designed combining pendulum controller and wheel controller. Disturbance observer is used to cancel disturbance effects. Simulation was carried out to prove the applicability of the proposed system.