The present study evaluated the influence of LIPUS on regeneration processes of bone defects below the critical size in irradiated and non-irradiated rabbit tibia. The study was based on a total of six white New-Zealand adult female rabbits. Apart from surgery to create bone defects on all tibiae, the following four treatments were randomly added on: (1) C group: only the surgical procedure was applied with no additional treatment, serving as the control, (2) R group, the irradiated side received 15 Gy in single dose, (3) US group, treated with LIPUS, and (4) R+US group, irradiated with 15 Gy and treated with LIPUS (n=6 defects per group). The surgery control samples showed 83.10% ± 17.79% of bone repair after 9 weeks, while the irradiated bone had significantly (p < 0.5) less regenerated during the same period (66.42% ± 29.36%). LIPUS treatment on irradiated bones performed a 79.21% ± 21.07% bone fill and could not significantly (p > 0.05) improve the response compared to the non-treated irradiated specimens. In the irradiated bones, ultrasound treatment produced only 3.89% less new bone compared to the untreated control group; this repair is insignificantly lower than the natural bone healing in the untreated control group. LIPUS treatment on non-irradiated bone, however, showed bone formations beyond the size defect (115.91% ± 33.69%), highly significantly different when compared to the control group or any irradiated group. It is noteworthy that the application of ultrasound to healthy bone produced highly significantly enhanced bone formations, with 36.70% more regenerated bone when comparing the same application on irradiated bone (79.21% ± 21.07%). LIPUS vibration stimuli may be considered as a promising complementary treatment approach in non-irradiated bone regeneration procedures to shorten the treatment and to enhance the bone healing. In irradiated bones, the effect of ultrasound application is less apparent and further studies are needed to refine the dynamics of the present results.