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A μIMU which consists of microelectromechanical systems (MEMS) accelerometers, gyroscopes and magnetometers has been developed for real-time estimation of human hand motions. Along with appropriate transformation and filtering algorithms, the μ IMU was implemented as a Ubiquitous Digital Writing Instrument (UDWI), which could interface with PCs in real-time via Bluetooth wireless protocol, to record the handwriting on any flat surface. However, because of the MEMS sensors' intrinsic biases and random noise such as circuit thermal noise, an effective calibration system that provides good reference measurement parameters must be developed to compare the output of the μIMU sensors to human hand motions. In this paper, we present our development of a method to calibrate three-dimensional linear accelerations and angular velocities of human writing motions measured from MEMS sensors through optical tracking techniques. In our experiments, English alphabets were written by the UDWI on a horizontal plane. The sensor output from the writing motions were transmitted wirelessly to a PC and the data were stored in the PC. Simultaneously, we recorded the pen-tip motion during the writing of each alphabet with a high-speed camera, which allowed us to exact the acceleration, velocity, and position of the UDWI's tip through appropriate optical-tracking algorithms. Then, the information is compared with the motion information obtained from the MEMS sensors in the UDWI. The motion data obtained from the high-speed camera are much more accurate, and hence could be used as reference motion data to analyze the performance of the UDWI, and eventually allows improvement of the UDWI performance.