Based on Orthogonal Frequency Division Multiplexing (OFDM), Orthogonal Frequency Division Multiple Access (OFDMA) has emerged as one of the prime multiple access schemes for broadband wireless networks, e.g. the IEEE 802.16 wireless MAN standard. While OFDMA is relatively simple to implement in the downlink, it is more challenging in the uplink as users must be aligned in time and frequency to maintain the orthogonality of the subcarriers. This paper proposes a novel method to estimate jointly timing and frequency offset for multi-user in the uplink OFDMA. The scheme is compatible with the ranging procedure of the current IEEE 802.16 standard. It is faster, requires less overhead and has lower complexity, compared to conventional estimation schemes.