Although extensive efforts have been dedicated to investigating the risk factors of low back pain in the workplace, a clear knowledge of their effects on the facet joint (FJ) mechanics is lacking. In this study, fourteen healthy participants performed dynamic lifting task with varying external load while a dynamic stereo-radiography system captured their lumbar motion continuously. The FJ kinematics in the lumbar spine were ascertained using a volumetric model-based tracking method. The FJ kinematics data from seven participants were processed and analyzed using non-parametric statistical test. The results indicated significant (p<0.05) effects of external load on the FJ flexion and superior-inferior translation at all segments, showing more consistent trends at the L2-L3, L3-L4, and L4-L5 joints during trunk flexion angles of approximately 20° and 40°. Findings of this study provide a preliminary but important foundation in elucidating facet-related injury mechanism due to strenuous exertions in workplaces.