In this paper, we study the behavior of the singular values of Hankel operators on weighted Bergman spaces A 2 ωϕ , where ω ϕ = e −ϕ and ϕ is a subharmonic function. We consider compact Hankel operators H φ , with anti-analytic symbols φ, and give estimates of the trace of h(|H φ |) for any convex function h. This allows us to give asymptotic estimates of the singular values (s n (H φ )) n in terms of decreasing rearrangement of |φ ′ |/ √ ∆ϕ. For the radial weights, we first prove that the critical decay of (s n (H φ )) n is achieved by (s n (H z )) n . Namely, we establish that ifand only if φ ′ belongs to the Hardy space H p , where p = 2 (1+β) 2+β . Finally, we compute the asymptotics of s n (H φ ) whenever φ ′ ∈ H p .