In this paper, Riemannian manifolds with a harmonic Weyl’s tensor are investigated. The problem of Riemannian manifolds classification with a harmonic Weyl’s tensor is considered to be complicated. Therefore, it is natural to study it in a class of homogeneous Riemannian spaces and, in particular, in a class of Lie groups with a left invariant Riemannian metrics. When the dimension equals to three theWeyl’s tensor is trivial. Therefore, there is a question of the Weyl’s tensor being harmonic in metric Lie groups with dimension greater than three. Four-dimensional unimodular Lie algebras of Lie groups with a left invariant Riemannian metrics and a harmonic Weyl’s tensor were studied by the authors of the paper. In the paper we study four-dimensional nonunimodular nondecomposable Lie groups with a left invariant Riemannian metrics and a harmonicWeyl’s tensor. Some methods with possible reduction of this problem to solution of the system of polynomial equations in Lie algebras are obtained. As a result of this classification, the Lie algebras with metric Lie groups that are not conformally flat, i.e. have non trivial Weyl’s tensor, are distinguished.