Abstract: Eddy current testing of deep-lying defects needs highly sensitive magnetic sensors at low frequency, such as flux-gate sensors, GMR/GMI sensors, and SQUIDs. We built a flux-gate ECT system and measured the parallel component of the magnetic field of the eddy current. The measured parallel component could be set zero by adjusting the position of the induction coil. In our previous work, the detection depth of 6061 aluminum alloy was 14 mm. In this work, we measured the magnetic field amplitude-frequency curves to find out the optimized frequency. The data were processed by FFT method and the number of measured points was increased in order to reduce the noise. As a result, the detection depth of 304 stainless steel was 20mm, and that of 6061 aluminum alloy was increased to 16mm. The depths of unknown defects can also be estimated by measuring the optimized frequency.