Since there is always an incorrect sign in the mass of the graviton in the so-called perturbation expansion approximation of de Sitter spacetime, the existence of a gravitational wave from the metric perturbation of de Sitter spacetime is doubtful. We try to take another way to start from the assumption that the cosmological gravitation wave equation should be both general covariant and conformal invariant. It is found that the so-called conformal gravitation is no longer part of the metric field and it has an effective mass of m_g =√R/6 =√2Λ/3 with the correct sign in de Sitter spacetime, though its intrinsic mass remains zero.