Abstract: The present study describes the development of a tri-axial accelerometer Physical Activity Monitoring system (PAMs) for the assessment of daily physical activity amplitude and frequency ranges and estimate energy expenditure. The PAMs is a wireless and small portable data-logger system that uses a selective and high range (±2.5 - ±10 g) tri-axial Microelectromechanical Systems (MEMS) accelerometer. In contrast with low range accelerometer data loggers in the market, PAMs has selectable sensitivity range (2.5-10 g) triaxial accelerometer capable of attachment of three external triaxial accelerometers in a 6x4x1.5 cm size and light weight (25 g). Physical activity monitoring system can be attached to the waist and external accelerometers can be strapped to the limbs to record three dimensions acceleration in multi locations. The PC software has been developed to demonstrate the recorded activities and save under personal database. Dynamic accelerations in each axis recorded on MicroSD card to be meaningful and interpretable for the researchers. It was laboratory tested for linearity and noise for the triaxial accelerometer. Noise RMS (0.1-1 k Hz) was 4.8 mVrms and power spectral density RMS (0.1-1 k Hz) was 350 μg Hz-2. Maximum Nonlinearity was 0.2 g. Test-retest reliability was very high for dynamic tests in three axes (r values ranging from 0.89 to 0.95). Test-retest experiments showed that the offset and the sensitivity of the accelerometer were equal for each measurement direction and remained constant on three measurement days.