Simulations are carried out for an athermal pile of particles in the presence of a gravity-like force. As the force increases, the pile structure rearranges through discrete avalanche events. The sizes of the avalanche events are characterized by the change in energy, and it is found that the avalanche size follows a power-law probability distribution with a power-law exponent of -0.96. These results suggest that the particle pile is in a self-organized critical state. (C) 2003 Elsevier Science B.V. All rights reserved.