The Cartan-Hartogs domains are defined as a class of Hartogs type domains over irreducible bounded symmetric domains. For a Cartan-Hartogs domain Ω B (µ) endowed with the natural Kähler metric g(µ), Zedda conjectured that the coefficient a 2 of the Rawnsley's ε-function expansion for the Cartan-Hartogs domain (Ω B (µ), g(µ)) is constant on Ω B (µ) if and only if (Ω B (µ), g(µ)) is biholomorphically isometric to the complex hyperbolic space. In this paper, following Zedda's argument, we give a geometric proof of the Zedda's conjecture by computing the curvature tensors of the Cartan-Hartogs domain (Ω B (µ), g(µ)).