Interaction force appearing in coalescence process of metal nano-clusters (atomic aggregates) is investigated by molecular dynamics method. Special elaboration is made in constructing a method by which atomic stress tensor calculated for each atom is capable of being visualized and made recognized three-dimensionally. With the conceptional help of principal directions, principal stresses, and stress ellipsoids, which are ordinarily used in continuum mechanics, the method of displaying both atomic migration and atomic stress state is developed. Though, even in isolated situation, metal nano-clusters have tensile stress in surface region which is the origin of surface tension, tensile stresses large in normal direction appear at the moment of coalescenceing of two clusters. After coalescence normal component is reduced and principal direction becomes tilt. It is also recognized that compressive stress is generated inside clusters and is propagated through the body.