In this paper, we consider the following two problems: Problem I. Given X E Rmxn, A=diag (λ1,•••,λm)>0, find A E BSRnoxn such that ‖AX-XA‖=min, where ‖•‖ is Frobenius norm, BSRnoxn is the set of all n x n bisymmetric nonneg-ative definite matrices. Problem II. Given A* ∈ Rnxn, find ALS ∈ SE such that ‖A*-ALS‖=inf ‖A*-A‖, AESE where SE is the solution set of problem I. The existence of the solution for problem I, Ⅱ and the uniqueness of the solution for Problem Ⅱ are proved. The general form of SE is given and the expression of ALS is presented.