Abstract: A Fisher discriminant analysis method was presented for the purpose of automatic detection of surface defects on steel ball. The eddy current testing method was utilized to detect the defects. Hilbert transform was employed for the envelopment demodulation analysis of the output signal. The features of the envelop signal was extracted and the classification algorithm based on Fisher criterion function was designed. The validity of the algorithm was verified and the threshold influence on the results was also discussed. The results showed that this method recognized the defects effectively after the threshold adjustment.