This paper discusses the rotation of the Earth or more generally of an extended body located in a gravity field in the framework of General Relativity. The authors work in the post-newtonian approximation (weak field) using the metric formalism developped by Damour, Soffel and Xu. This metric is used to derive the equations of the spin axis motion with respect to the kinematically (and dynamically) nonrotating system. The authors study the rotation i.e. the so called geodesic precession and nutation between these two kinds of system, and calculate analytically and numerically the post-newtonian nutations up to 10^-6mas order.