A new inspection technique for complex mechanical structures is proposed in this paper, where a fuzzy inference system carries out structural inspection. The inputs to the fuzzy inference system are the elements of a fault signature, an array of numbers prepared with use of below 5 kHz resonance frequencies of faultless and a number of faulty specimens.Advantage: Below 5 kHz resonance frequencies are easier and less expensive to obtain compared to higher frequency ones.Limit: Due to high expenses of experiments, reliable finite element models were alternatively used to obtain resonance frequencies of the faulty specimens.Results: The developed fuzzy inference system in this research accurately located an under-surface fault in an engine cylinder block.