This paper estimates the yields of DNA double-strand breaks (DSBs) induced by ultrasoft X-rays and uses the DSB yields and the repair outcomes to evaluate the relative biological effectiveness ( ) of ultrasoft X-rays. We simulated the yields of DSB induction and predicted them in the presence and absence of oxygen, using a Monte Carlo damage simulation (MCDS) software, to calculate the . Monte Carlo excision repair (MCER) simulations were also performed to calculate the repair outcomes (correct repairs, mutations, and DSB conversions). Compared to Co γ-rays, the values for ultrasoft X-rays (titanium K-shell, aluminum K-shell, copper L-shell, and carbon K-shell) for DSB induction were respectively 1.3, 1.9, 2.3, and 2.6 under aerobic conditions and 1.3, 2.1, 2.5, and 2.9 under a hypoxic condition (2% O ). The values for enzymatic DSBs were 1.6, 2.1, 2.3, and 2.4, respectively, indicating that the enzymatic DSB yields are comparable to the yields of DSB induction. The synergistic effects of DSB induction and enzymatic DSB formation further facilitate cell killing and the advantage in cancer treatment.