Abstract. Let M be an n-dimensional compact hypersurface with constant scalar curvature n(n−1)r, r > 1, in a unit sphere S n+1 (1). We know that such hypersurfaces can be characterized as critical points for a variational problem of the integral M HdM of the mean curvature H. In this paper, we first study the eigenvalue of the Jacobi operator J s of M . We derive an optimal upper bound for the first eigenvalue of J s , and this bound is attained if and only if M is a totally umbilical and non-totally geodesic hypersurface or M is a Riemannian product S m (c) × S n−m ( √ 1 − c 2 ), 1 ≤ m ≤ n − 1.