The characteristic measurements of permanent magnet motors, such as torque-speed, power, and efficiency, are essential during manufacturing. Dynamometer equipment has been extensively used in motor characteristic measurements. This approach is known to be time-consuming and may not be suitable for on-line product quality control. This paper presents a novel method for obtaining the moment of inertia without using external mechanical load, which is capable of rapidly measuring the torque-speed characteristics of permanent magnet motors. The experiment results demonstrate the effectiveness of the proposed method, which can obtain the required measurements within 4 seconds. The measurement error can be less than 4% in comparison with that using the dynamometer. The proposed method aims to be adopted in motor mass production for on-line quality control.