This paper aims to propose an improved probe for remote field eddy current testing (ECT) method with a ferromagnetic core inserted into coils, and to verify the effect of the improved detection sensitivity. The remote field eddy current testing method which installs an excitation coil and a search coil inside a pipe is known to be effective in detecting external surface defects in ferromagnetic pipes used in petrochemical or steel industries. In previous research, the phase angle between the flux density due to the excitation current and that due to the eddy current inside the pipe, when a magnetic field of several hundred Hz is applied to the excitation coil, was analyzed. It has been clarified that the detection signal sensitivity is reduced because of the phase angle approaches 180 degrees and cancel each flux density out. In this research, an improved remote field ECT probe with ferromagnetic core inserted in coils is proposed to increase the detection signal sensitivity. The measured sensitivity of the proposed probe is increased compared with the conventional one. The mechanism of different shapes and materials are analyzed in detail by using 3D nonlinear eddy current analysis using finite element method (FEM). It was found that that when a core of S45C was inserted in the excitation coil, the phase angle change became larger at the defect position because the phase angle of the flux density due to excitation current is delayed by eddy current in the core which affected the sensitivity improvement.