Fibroblasts are cells that secrete components of the extracellular matrix in fibrillar tissues such as ligaments. During healing ligament process, fibroblasts proliferate and migrate to the injury area in order to produce and remodel collagen type III and type I. Therapeutic physical agents used in physiotherapy are used to improve these processes. Ultrasound is a therapeutic physical agent with high controversy. Experimental studies in humans have not shown effectiveness in the treatment of ligament injuries. In vivo and in vitro studies show discrepancies in their results in terms of cell proliferation rate and production of extracellular matrix components in various soft tissues. However, its use is common in rehabilitation centers. Therefore, the aim of this paper is to describe the effect of ultrasound on cell proliferation. An experimental model in vitro monolayer culture of mouse embryonic fibroblast NIH-3T3 exposed to ultrasound of 1MHz pulsed, 1Watts/cm2 to 50% was used. One of the treatments was exposed to pulsed ultrasound, while the other one, as a control group, was not. The results showed an increase in cell proliferation in the treatment group through the growth curve. In conclusion, this experimental model is the basis for future research about mechanical and biological fibroblasts properties isolated from knee collateral ligaments; it also provides knowledge in cellular levels about effectiveness of ultrasound on ligament healing.