Focusing on space-frequency transmit diversity OFDM transmission through frequency selective channels, this paper proposes a computationally efficient, maximum a posteriori(MAP) channel estimation algorithm. The algorithm requires a convenient representation of the discrete multipath fading channel based on the Karhunen-Loeve orthogonal expansion and estimates the complex channel parameters of each subcarriers iteratively using the Expectation Maximization(EM) method. In order to explore the performance, the closed-form expression for the average symbol error rate (SER) probability is derived for the zero-forcing equalizer. Furthermore, the MAP channel estimator's performance is studied based on the evaluation of the modified Cramer-Rao bound. Simulation results confirm our theoretical analysis, and illustrate that the proposed algorithm is capable of tracking fast fading and improving overall performance.