In this paper we investigate the existence of positive solutions of a nonlinear biharmonic equation under Dirichlet boundary condition in a bounded open set ${Omega}$ in $mathbf{R}^n$, i.e., $${Delta}^2u+c{Delta}u=bu^{+}+s;in;{Omega},\u=0,;{Delta}u=0;on;{partial}{Omega}$$.