In this work we de ne the concept of relative flatness of a system with respect to a subsystem. The subsystem associated to a set of outputs of a system is constructed, and called here output subsystem. It is shown that the relative flatness of a system with respect to the output subsystem implies the flatness of the corresponding implicit system obtained by setting these outputs to zero. A sufficient condition of relative flatness based on a relative derived ag is presented. Based on these results, a sufficient condition for the flatness of a class of nonlinear implicit systems is obtained.