Permanent magnet machines are suitable for Hybrid Electric Vehicle (HEV) and Electric Vehicle (EV) application because of its superior torque density, high efficiency and small size. Especially, Interior Permanent Magnet Synchronous Motor (IPMSM) is an appropriate type because an air-gap flux density can be maximized and reluctance torque is enhanced using the saliency of rotor shape. However, a leakage flux generated in a magnetic rib which circulates inside the rotor and causes the decrease of the air-gap flux density. In this paper, an improved rotor structure is proposed where the magnetic ribs are removed and centre bridges are added for the mechanical stiffness. The air-gap flux density increased because two leakage paths are reduced to one path which are the magnetic ribs and the centre bridge respectively. In addition, the mechanical strength are analysed and compared. Finally, the improved model is manufactured and test is conducted for the verification.