Previous studies have indicated that low intensity ultrasounds could accelerate the repair of fibula fracture and facilitate the proliferation of osteoblasts. To further investigate the effect of low intensity ultrasounds, two different frequency of ultrasound, 1 MHz and 3 MHz, were applied to osteoblasts. The cells were stimulated for a typical 20% duty cycle of ultrasound with various intensities ranged from 50 to 150 mW/cm2 (ISATA) for 3 minutes once daily for 6 days. The cellular responses, in terms of cell number and morphological change, associated with ultrasound stimulations were estimated using a hemocytometer and microscopic morphology in which cells were stained with trypan blue. Results showed that the proliferation rates of osteoblasts for particular stimulated group are larger than those of control groups. The largest proliferation rate corresponds to those cells culture at 24 hours after seeding. Ultrasound increased approximately the cell proliferation proportional to ultrasonic intensity at which the exposure of 100 mW/cm2 intensity of 1 MHz ultrasound 1.1 fold, and 50 mW/cm2 intensity of 3 MHz ultrasound 1.2 fold than that of the control group. These finding suggest that the growth of cell may be controlled by appropriate ultrasonic mechanical stress.