This paper addresses the problem of building a compact representation of objects which enables to define a similarity measure. The key idea is to provide a mean to extract and represent the relevant information of an object. By integrating the minimum length description to an informational similarity metric, a new informational measure is proposed based on models. First, we make the strong assumption that for any object x there exists a model Mx such that K(x) = K(x | M x) + K(Mx)