Abstract: In order to overcome the difficulty of nonlinearity and ill-posedness of the defect quantitative identification with the single-side infrared inspection, a new algorithm(Chaos-LM hybrid algorithm),which combined the chaos algorithm with the Levenberg-Marquardt method,was introduced in this paper. And a new concept of relative sensitivity coefficient was also brought forward to analyze the defect inspectability. Through analyzing the relative sensitivity coefficient, it was found that the inspectability of different parameters describing the size and position of the defect was different, and the inspection carried out at the time with maximum surface temperature difference was more desirable than that in steady heat transfer state. Both of the transient and steady inspections were tested by a series of numerical experiments, and the experimental results are consistent with the analysis of relative sensitivity coefficient. Besides, the numerical experiments have certified the effectiveness of the Chaos-LM hybrid algorithm, and show that the identification results are stable and globally optimal.