Abstract: Frequency-domain equalization for single-carrier transmission systems provides an attractive design alterative to time domain equalization because of more efficient calculation. In this paper an Iterative Block Decision Feedback Equalization (IB-DFE) scheme for communication systems over UnderWater Acoustic (UWA) channels with long delay and Doppler frequency shifts is researched and a frequency domain channel estimation algorithm Jointing iterative Equalization and Channel Estimation (JECE) is proposed. The performance of JECE algorithm is evaluated and compared. The influences of different data parameters on the performance of IB-DFE are analyzed and the computational complexities of IB-DFE are calculated. Numerical results show that the proposed JECE algorithm is more adaptive to the time-varying UWA channels. Compared with Time Domain Decision Feedback Equalization (TD-DFE), IB-DFE has more than 3.6 dB signal-to-noise ratio gain and 19% complexity gain and these gains will extend as the increasing of delay and Doppler frequency shifts of UWA channels.