In this paper, we study a new class of Finsler metrics, F = αφ(b 2 , s), s := β/α, defined by a Riemannian metric α and 1-form β. It is called general (α, β) metric. In this paper, we assume φ be coefficient by s and β be closed and conformal. We find a nessecary and sufficient condition for the metric of relatively isotropic mean Landsberg curvature to be Berwald.