The preparation of copper oxalate fine particles was carried out by using an emulsion liquid membrane (W/O/W emulsion) to control the size of particles. Copper ions were extracted from the external water phase and stripped into the internal water phase, and precipitated as copper oxalate. Particles of 0.18 - 0.50 mu m in diameter, which were aggregates of primary particles of about 8 nm, were obtained. The particle size decreased with increasing volume ratio of organic membrane phase to internal water phase of the W/O emulsion (O/A ratio). The number of internal water droplets in which the Cu ions are distributed and precipitated is greater at O/A ratio=1 than at O/A ratio=0.5, since the emulsion droplets are dispersed well in the external water phase at O/A ratio=1. Therefore, the number of Cu ions moved into one droplet, and thus, the number of primary particles are greater at O/A ratio=0.5 than those at O/A ratio=1, resulting in the formation of greater particles by aggregation of the primary particles. The simulation based on the model of transfer mechanism of Cu ions through the emulsion liquid membrane was also carried out, in order to confirm the reasonability of the proposed mechanism of particle formation.