Among materials-related applications of supercritical fluid it may be said that RESS(Rapid Expansion of Supercritical Fluid Solutions) process is promising to produce particles which have small sizes and narrow size distributions without residual solvent in the produced particles. In this work supercritical carbon dioxide and 2-naphthol have been selected and the effects of various experimental variables in the RESS process are investigated. The experimental variables include capillary diameter and length, extractor temperature and pressure, crystallizer pressure and preheater temperature. The results show that the diameter of the particle becomes larger with increasing extractor pressure and temperature although the particles size are relatively insensitive to the temperature of preheater. Larger particles can be obtained with increasing crystallizer pressure. In terms of the effects of capillary dimension the particle size decreases with increasing capillary length, while ti increases with increasing capillary diameter. Moreover, when the particle diameter is related to the degree of supersaturation, we can get excellent correlations between them.