The transesterification of soybean oil and methanol to prepare biodiesel using H2SO4 and solid acid (NaHSO4) as catalysts was conducted in CO2-expanded liquids (CXLs). The aim of adding CO2 in the system is to intensify the reaction, and then to shorten the reaction time and decrease the methanol consumption in the traditional acid catalysis method or decrease the high temperature and pressure in the supercritical methanol method. The parameters affecting the fatty acid methyl esters (FAME) yield, such as pressure, temperature, and the molar ratio of methanol to soybean oil (M/O ratio), as well as catalyst amount were investigated. The results indicated that a complete oil conversion happened at 10 MPa, 70 degrees C (reaction time 6 h) and 80 degrees C (reaction time 4 h) with M/O ratio of 12:1, and H2SO4 amount of 4%, while for NaHSO4 catalytic reactions, the FAME yield was 80.94% after 6 h at 10 MPa, 90 degrees C, and the M/O ratio 9:1, NaHSO4 amount of 5% (based on oil weight). The underlying reasons why CO2 enhances the transesterification of oil and methanol were highlighted.