Abstract Objective: The purpose of this study was to evaluate the effect of an 830-nm GaAlAs diode laser operating at output powers of 40, 60, 80, and 100?mW and energy density of 30?J/cm(2) on the repair of partial calcaneal tendon ruptures in rats. Methods: A partial tendon rupture was induced in all animals, which were treated with laser irradiation for 5 consecutive days. Six days after injury, the injured tendons were removed and examined by polarized light microscopy. Collagen fiber organization was evaluated by birefringence measurements, and collagen content was determined by Picrosirius Red staining. Results: It was observed that the higher the output power (60-100?mW) the greater the amount of type III collagen (p<0.01). The amount of type I collagen was significantly greater (p=0.05) in the 80?mW group than in the control group (sham stimulation). A non-statistically significant improvement in the realignment of collagen fibers was observed in the irradiated groups. Conclusions: Low-level laser therapy resulted in significantly greater amounts of type III collagen (output powers of 60?mW or more) and type I collagen (output power of 80?mW), however, no significant differences between groups were found in the realignment of collagen fibers.