In this paper, an efficient and practical design for Doppler spread estimation in mobile OFDM systems are described. A channel auto-correlation function algorithm is proposed for Doppler spread estimation. The Doppler spread estimator is based on finding the auto-correlation function of time domain channel estimates over several OFDM symbols. The estimation performance of the estimator is evaluated via computer simulation using national standard digital television system DTMB signal transmission format. Simulation results show that the proposed Doppler spread estimator can provide high estimation accuracy for a wide range of Doppler spread. Moreover, the proposed algorithm relies on periodic channel estimate and can also be applied into other OFDM systems with known pilot symbols inserted within the data fields.