The aim of this paper is to present a simple but comprehensive model for the dynamical response of cracked rotor. The mechanical system is made of two rigid bars connected with a nonlinear spring. The two bars represent the uncracked parts of the rotor, and, the nonlinear spring represents the cracked section. The breathing mechanism of the crack is taken into account by considering special periodic variation of the global stiffness of the system. The differential equations system is soled using the harmonic balance method. Some possibilities for early crack detection are established.