The “black-box” modeling of dc-dc converters has always been an attractive topic and widely used in engineering practice. However, in order to obtain un-terminated model of dc-dc converter, it had to be removed from the original environment and connected to the high bandwidth voltage source and current sink, for the purpose of easier decoupling of source and load dynamics. This paper addresses an on-line identification procedure, where dc-dc converter can remain in the original environment, and after the frequency response characteristics are obtained, system can be decoupled through the linear transformation of the g-parameter model matrix. The verification and validation of the proposed technique will be both presented in this paper.