Abstract: This paper presents the deducing procedures and formulations of rigorous solutions and approximate solutions (i. e. when taking transmitter coil as point magnetic dipole) of induction voltage of dual coil in homogeneous medium. Relative errors of amplitude ratio and phase shift caused by the approximation are calculated by taking a simple radio-frequency logging tool as an example. Other conventional A.C. (alternatingcurrent) logging tools are studied with the same method.Study results show that, as to conventional logging tools,this kind of errors can be ignored.