The instability and non-linear dynamics of a slender rotating shaft with a rigid disk at the mid-span are analyzed. The shaft is simply supported at both ends and is made of a viscoelastic material. The stability criteria are determined from the linear equations of motion based on the small strain assumption. The bifurcation of the double zero eigenvalue point on the stability boundaries in the parametric space is analyzed by using center manifold theory on the non-linear equations of motion, for which a large transverse displacement of the shaft is assumed. Analytical expressions for the radius of synchronous whirling and the radius and precession rate of non-synchronous whirling near the double zero eigenvalue point are obtained explicitly. The behaviors of the parametric points away from the stability boundaries are analyzed numerically. The general effects on the precession rate for these points are somewhat different from those for the parametric points in the vicinity of the double zero eigenvalue.