Objective To investigate the effect of mechanical loading with different magnitudes on the proliferation, differentiation and activity of preosteoclasts and osteoclasts. Methods One group of RAW264.7 preosteoclastic cells cultured in osteoclast inductive medium were subjected to the cyclic tensile strain for three days, and then cultured for four days; the other group of RAW264.7 cells were induced in osteoclast inductive medium for four days to be osteoclasts, then subjected to the cyclic tensile strain for three days. Results Under the tensile strain at different magnitudes, the proliferation variations in two groups of RAW264.7 cells were approximately identical, but changes in the activities of tartrate-resistant acid phosphatage (TRAP) and numbers of TRAP-positive multinucleated cells (osteoclasts) in the two groups were significantly different. Under the moderate tensile strain (2 500 με), the reduction of TRAP activity and osteoclasts number were both the highest in the first group, and both the lowest in the second group. Conclusions The influence of different tensile strain on osteoclast differentiation and osteoclastic activity of preosteoclasts in early differentiation is different to that of the preosteoclasts already differentiated into osteoclasts.