In this paper we present theoretical analysis which gives the possibility to determine optimum excitation frequencies in eddy current examination of thick aluminum plates. A computer controlled non‐destructive testing system and a probe with a magnetoresistive sensor were utilized in laboratory tests. Experiments with specimens containing hidden cracks were carried out for a wide spectrum of frequencies. The optimum operating frequencies achieved from the experiments are in a very good agreement with those obtained by using theoretical analysis. Application of the optimum frequencies makes it possible to detect shallow cracks located on the reverse side of the specimens. Detection of a 15 &percent; crack in a 10 mm thick plate made of aluminum and a 20 &percent; crack in a 20 mm thick plate has been confirmed. © 2003 American Institute of Physics