This paper proposes a new scheme based on information theoretical criteria to estimate the channel-impulse-response (CIR) length, whose accurate estimate represents one of the most important tasks for the realization of OFDM systems with adaptive guard interval. The difference between the statistical characteristics of the additive noise and the mobile radio channel is exploited for this task. In the proposed algorithm, the calculation of the autocorrelation matrix and their eigenvalues are not required. Therefore, the complexity of the proposed method is low. A modified penalty function is introduced to make the criteria more stable. Performance of the proposed method is evaluated for HiperLAN/2 channels. The simulation results show that the proposed algorithm is robust and delivers a precise estimate for the CIR length even at sever noisy environment with as low signal-to-noise ratio (SNR) as 0 dB. It delivers therefore reliable information for adaptive guard interval (GI) based OFDM systems and channel estimation