Let x : M → R n+p (c) be an n-dimensional compact, possibly with boundary, submanifold in an (n + p)-dimensional space form R n+p (c). Assume that r is even and r ∈ {0, 1, . . . , n − 1}, in this paper we introduce rth mean curvature function S r and (r + 1)-th mean curvature vector field S r+1 . We call M to be an r-minimal submanifold if S r+1 ≡ 0 on M, we note that the concept of 0-minimal submanifold is the concept of minimal submanifold. In this paper, we define a functional J r (x) = M F r (S 0 , S 2 , . . . , S r )dv of x : M → R n+p (c), by calculation of the first variational formula of J r we show that x is a critical point of J r if and only if x is r-minimal. Besides, we give many examples of r-minimal submanifolds in space forms. We calculate the second variational formula of J r and prove that there exists no compact without boundary stable r-minimal submanifold with S r > 0 in the unit sphere S n+p . When r = 0, noting S 0 = 1, our result reduces to Simons' result: there exists no compact without boundary stable minimal submanifold in the unit sphere S n+p .