The thermal effects induced by high-power diode pumping in the Nd : YVO_4 slab laser crystal are numerically modeled. Using three-dimensional (3D) finite volume method, an accurate numerical solution based on heat transfer equation is achieved. The transient temperature distribution curves and the steady state 3D temperature distributions of the diode double end-pumped, uniformly facepumped and the periodical face-pumped Nd : YVO_4 slab are presented.