We consider the fourth order problem ∆ 2 u = λf (u) on a general bounded domain Ω in R n with the Navier boundary condition u = ∆u = 0 on ∂Ω. Here, λ is a positive parameter and f : [0, a f ) → R + (0 < a f ∞) is a smooth, increasing, convex nonlinearity such that f (0) > 0 and which blows up at a f . LetWe show that if u m is a sequence of semistable solutions correspond to λ m satisfy the stability inequalitythen sup m ||u m || L ∞ (Ω) < a f for n < 4α * (2−τ+)+2τ+ τ+ max{1, τ + }, where α * is the largest root of the equationIn particular, if τ − = τ + := τ , then sup m ||u m || L ∞ (Ω) < a f for n ≤ 12 when τ ≤ 1, and for n ≤ 7 when τ ≤ 1.57863. These estimates lead to the regularity of the corresponding extremal solution u * (x) = lim λ↑λ * u λ (x), where λ * is the extremal parameter of the eigenvalue problem.