The aim of this study was to develop a sensor based gait assessment algorithm and investigate the concurrent test-retest (split-half) reliability and inter-instrument reliability of inertial measurement units (IMUs). Nine healthy young adults (age = 28±5 years) performed 10 walking trials at self-selected pace over a 15 m long walkway in the locomotion laboratory. Their accelerations and angular velocities were measured and an automated algorithm was built to extract gait parameters. For each walking trial, data was simultaneously collected using the IMU called Technology Enabled Medical Precision and Observation (TEMPO) and Qualisys infrared camera system with force plates, to determine its reliability. Test-retest reliability of the TEMPO system was measured by ICC (3,1) and inter-instrument (between camera system with force plate and TEMPO) reliability was measured by ICC(2,1). Test-retest reliability of the TEMPO system was excellent for foot swing angle, step length, and stride length (ICCs between 0.79 and 0.93), fair to good for required coefficient of friction (RCOF) and all the temporal gait parameters except right swing time (ICCs between 0.41 and 0.73), and poor for right swing time (ICC of 0.03). Left stance time was the only parameter that had high inter-instrument reliability (ICC of 0.87) and double support time and right stance time with fair reliability (ICCs of 0.47 and 0.55), whereas other parameters had poor reliability (ICCs between <0.001 and 0.25). The results of this study demonstrated that the automated gait assessment algorithm using the TEMPO system is highly reliable, yet it needs to be improved because of the low levels of agreement with the laboratory based gait analysis systems.