This paper describes an optimal stator-current control strategy for an Integrated Starter-Alternator (ISA) prototype. Based on the applications of hybrid electric vehicles with wide constant power speed range, this paper investigates the various operation stages for the interior permanent magnet machine, including the maximum torque per ampere (MTPA), flux weakening control and maximum output power stage. A control algorithm was proposed to optimize the stator-current trajectories. Through the algorithm, the d-axis and q-axis components of the stator current were driven on the optimal trajectories to produce the desired torque and to maintain system stability under the voltage limit and current limit. The system takes full advantage of the capacity of the driving converter, and the maximum output torque was generated by the motor at any time. The reliability and dynamic performance of system were rather satisfactory. Energy optimization was achieved. The results from simulation and the real platform experiment verify the feasibility and advantages of the system.