Background: Range of Motion (ROM) of the joint is one of the elements that affect musculoskeletal system function. A valid and reliable method should be used to measure and record the ROM measurements. Purpose: This study was conducted to test the criterion related validity and the intra-rater reliability of rebee wearable sensor in measuring Active Range of Motion (AROM) of knee joint flexion and extension . Methods: Forty normal participant's age ranged from 30 to 45 years and their Body Mass Index (BMI) from 19 to 25 kg/m2 involved in one group. The Digital Goniometer and rebee wearable sensor were utilized to measure AROM of knee joint flexion and extension. Results: showed that measurement of AROM of the knee joint flexion with rebee sensor was significantly correlated with the measurement of digital goniometer for AROM of knee joint Flexion (p-value=0.001).The intra-rater reliability using the Intra-class Correlation Coefficient (ICC) showed that there was a high reliability of rebee wearable sensor in measurement the AROM of knee joint Flexion ,but there was a low reliability of rebee wearable sensor in measurement the AROM of knee joint Extension when compared with measurements for AROM of knee joint using digital goniometer. Conclusions: Rebee wearable sensor is a valid instrument for measurement of AROM of knee joint flexion, but not valid for measurement of AROM of knee joint extension.Regarding reliability rebee wearable sensor has high reliability of measurement of AROM of knee joint flexion, but low reliability for measurement of AROM of knee joint extension.