This paper presents the performance evaluation of our wireless miniature Inertial Measurement Unit (IMU) WB-4 by compared with the Vicon motion capture system. In particular, a magnetic field calibration method is introduced to improve the sensor orientation estimate accuracy. The WB-4 IMU primarily contains a motherboard for motion sensing, a Bluetooth module for wireless data transmission with PC, and a Li-Polymer battery for power supply. The motherboard is provided with a 32-bit microcontroller and 3-axis miniaturized MEMS accelerometer, 3-axis gyroscope and 3-axis magnetometer to estimate the sensor orientation based on an extended Kalman filter algorithm. In our previous research of WB-4 IMU performance evaluation, the factory calibration parameters of the magnetometer were used for the sensor fusion, which resulted in a higher error on the yaw angle in respect to roll and pitch. This study presents a magnetic calibration method for overcoming that limitation. The experimental results showed that the wireless WB-4 IMU could achieve better orientation performance in all the directions after the implementation of the magnetic calibration method. The yaw angle accuracy was significantly improved from previous error 5.46 degree to 1.77 degree.