Q-curvatre is a natural generalization of Gaussian curvature on surfaces. Originally, it was defined only for 4-manifolds. Due to works of Paneitz and Branson, this definition can be extended to arbitrary manifolds with dimension at least three from analytical aspects. In this talk, we investigate Q-curvature from a geometric aspect. In particular, we present a volume comparison theorem with respect to Q-curvature for strictly stable positive Einstein manifolds. This is a joint work with Huang Yen-Chang and Lin Yueh-Ju.