Specific Emitter Identification (SEI) identines radio emitters of interest using only external signal feature measurements. SEI can be used for intelligent gathering, network intrusion detection, battle field management, etc. Although abundant researches have been well developed, most of them are not associated with civil commercial products but radars. In this paper, we propose a method which estimates the power series coefficients of nonlinear devices in the Radio Frequency (RF) front end by observing the spectral regrowth. This external feature is unique due to different system designs and fabrication randomness. The performance of the technique is analyzed and experimented under AWGN channel based on an OFDM system, IEEE 802.11a/g, in both fixed and various transmitted power scenarios.