In this paper, we propose a new dynamic frequency assignment scheme for femtoCells in OFDMA networks. The femtoCell base stations are small cellular base stations specially designed to improve the poor indoor coverage and increase the system capacity for cellular and wireless broadband networks. However, these femto-base stations cause harmful interference to the macroCells. Therefore, the optimization of the radio resource allocation to femtoCells is essential to decrease the femtoCell to macroCell interferences and to offer better QoS for both femtoCell and macroCell users. Our dynamic frequency assignment scheme is based on sharing the same radio resources with macroCells which avoids resources' waste while condemning particular channels to femtoCells' deployment. Moreover, our scheme assigns the “quietest” channels to femtoCells according to the received power level. Hence, femto-BSs and macro-BSs radio resources' share could be achieved without introducing serious interference. Simulation results show the efficiency of our frequency assignment scheme. The average achievable throughput of both macroCell and femtoCell users is increased by more than 20% when compared to that obtained under a common assignment scheme.