Existing methods of measuring loop gain of digitally controlled DC/DC converters mainly fall into two categories, namely the analog signal disturbance method (ASDM) and the digital signal disturbance method (DSDM). However, the loop gains obtained from these methods do not totally agree with each other. Through rigorous theoretical analysis, this paper has derived the expressions of open-loop gains obtained from these two methods, whose accuracy has been verified by simulation. Also, this paper has put forward that the difference in signal that enters into the sampler results in the variation of the transfer function of the sampler, and further induces the difference between the open-loop gains obtained from the ASDM and the DSDM.