This paper contains calculations about propellant slosh for an upper stage rocket. The liquid fuel is modeled by using an equivalent mechanical model, from this point of view the stability of this rocket is examined by means of mass-spring model. In theory, a nonlinear mathematical model is derived and nonlinear feedback control laws are used. Specifications of the Attitude Vernier Upper Module (AVUM) upper stage rocket Vega are chosen for simulations. The results of this study are obtained for first four slosh modes.