Particle size distribution is described by general dynamic equation (GDE). A new multi-Monte Carlo (MMC) method is promoted to solve GDE for simultaneous particle coagulation and breakage. MMC method is based on "time-driven" Monte Carlo technique and conserves constant number of fictitious particles and constant volume of computational domain with the evolution of time. Firstly, MMC method is described in details, which includes the scheme of simultaneous coagulation and breakage, the introduction of "weighted fictitious particle", the setting of time step, the judgment of the occurrence of coagulation and breakage event, the choice of fictitious coagulation partner, and dealing with the consequence of particle coagulation and breakage event. Then MMC method is used to simulate four kinds of special cases in which complete or partial analytical solutions exist; the simulation results of MMC method for GDE agree with analytical solutions well, which proves that MMC method has high and stable statistical precision. (c) 2005 Elsevier B.V. All rights reserved.