By using a pilot sample from a selected time-slot in a time-domain orthogonal frequency division multiplexing (OFDM) block, we present a simple maximum-likelihood (ML) scheme for frequency tracking in OFDM systems over frequency-selective channels. The frequency offset estimator thus obtained is found to become unbiased and its mean square error (MSE) will approach the Cramer-Rao bound (CRB) as signal-to-noise ratio (SNR) is increased. Moreover, the selection of time slot can determine a trade-off between tracking accuracy and tracking range.