In this paper, we focus on extending the subspace identification to the class of linear periodically time-varying (LPTV) systems. The Lyapunov-Floquet transformation is first applied to the system’s state-space model in order to get the monodromy matrix (MM) and, thus, a necessary and sufficient condition for system stability. Then, given two successive covariance-driven Hankel matrices, the MM matrix is extracted by some calculus of a simultaneous singular value decomposition (SVD) and a least square optimization. The method is illustrated by a simulation application to the model of a hinged-blades helicopter.