In this paper, we investigate the existence of positive solutions for the ellipticequation $\Delta^{2}\,u+c(x)u = \lambda f(u)$ on a bounded smooth domain $\Omega$ of $\R^{n}$, $n\geq2$, with Navierboundary conditions. We show that there exists an extremal parameter$\lambda^{\ast}>0$ such that for $\lambda< \lambda^{\ast}$, the above problem has a regular solution butfor $\lambda> \lambda^{\ast}$, the problem has no solution even in the week sense.We also show that $\lambda^{\ast}=\frac{\lambda_{1}}{a}$ if$ \lim_{t\rightarrow \infty}f(t)-at=l\geq0$ and for $\lambda< \lambda^{\ast}$, the solution is unique but for $l<0$ and $\frac{\lambda_{1}}{a}<\lambda< \lambda^{\ast}$, the problem has two branches of solutions, where $\lambda_{1}$ is thefirst eigenvalue associated to the problem. پرونده مقاله