OFDMA (Orthogonal frequency division multiple access) based systems have been considered as a candidate for future wireless communication systems. In such a system, it is important to provide high data rate services for users with high mobility. For this purpose, decision-directed channel tracking (DDCT) techniques have been investigated for OFDM systems in high mobility environments. The performance of the DDCT in OFDMA systems depends on the assigned channel condition, and therefore it is important to allocate a proper channel to each user. The purpose of this paper is to propose an adaptive channel assignment method which enhances the DDCT improvement in an OFDMA system, where the former numbered resource blocks (RBs) in OFDMA frame are preferentially allocated to the users moving at a high speed. Computer simulation results show that the proposed method is effective in improving throughput performance of OFDMA systems in a high Doppler frequency condition.