Recently, with the development of home appliances and industrial equipment, most automation equipment has been used in industry. The most common equipment used is the motor, which requires accurate and efficient control. Additionally, motor operation requires rotor angle and speed information, which is obtained using an encoder and resolver. However, the sensor has drawbacks, including high cost, increased volume, and sensitivity to the environment. Therefore, many studies have recently been conducted on sensorless controls. In this paper, the proposed full-order flux observer method is based on flux estimation. Without sensors, the rotor angle and speed information can be obtained using d- and q-axes current and flux. However, the estimation performance of the full-order flux observer is reduced by the gain structure in the low-speed range. This paper proposes a full-order flux observer with an improved gain. The improved gain has stable estimation performance in the low-speed and high-speed ranges, which makes accurate speed control possible at low and high-speed. The proposed algorithm is verified through an experiment using a surface permanent magnet synchronous motor (SPMSM).