We investigate the force between two walls immersed in a Lennard-Jones liquid by means of a grand canonical Monte Carlo method. We consider two models (the attractive and repulsive interaction potential) for the interaction potential. In the case of the attractive interaction potential, a layer structure is formed in the Lennard-Jones particle system. The structure causes the force between the two walls, which oscillates around zero. In the case of the repulsive interaction potential, the force becomes weakly attractive for low chemical potential and repulsive for high chemical potential. (C) 1999 Elsevier Science B.V. All rights reserved.