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