Symbol guard interval is almost indispensable to deal with ISI caused by channel delay spreading in wireless OFDM transmission systems. However, the length of guard interval is usually fixed in conventional OFDM system, which decreases the power and spectrum efficiency considerably, especially in a mobile environment. To solve this problem, a novel conceptual adaptive OFDM system with variable guard interval (AOFDM-VGI) is proposed in this paper. This system first utilizes the preamble and pilot sub-carriers of each OFDM packet burst to estimate the channel RMS delay spread, and then the result is fed into the guard interval controller. Then, the controller uses a criterion to calculate the guard interval and control the OFDM transmitter. For a typical simulation model as illustrated in this paper, this system can transmit a data rate 11.5 Mb/s higher and consume a power 0.65 dB less than the conventional non-adaptive counterpart without sacrificing the BER performance.