Dynamic voltage and frequency scaling (DVFS) is an effective method for controlling both energy and performance of a system. Since the increasing rate of radiation-induced transient faults depends on operating frequency and supply voltage, DVFM techniques are recently shown to have compromising advantages on electronic system reliability. Therefore, ignoring the effects of voltage scaling on fault rate could considerably decrease the system reliability. In this paper we propose a formula for accurate analytic modeling of the soft error rate of a system in which the frequency and voltage are scaled using a DVFS algorithm. The simulation experiments show that, compared with other published models, the results using proposed model are 30% closer to the results from SER estimation algorithm which considers electrical and logical masking for ISCAS85' circuits.