Esterification of free n-3 polyunsaturated fatty acid (PUFA) with enriched n-6 triglyceride (TG) was catalyzed to modify the desired structured lipid with n-3/n-6=4 by using lipase under supercritical carbon dioxide (SCCO2)- Comparing four different types of lipases, 1,3-specific lipase from Mucor miehei has the highest degree of incorporation under 10 wt.% loading amount of total substrates. The optimal operating parameters under 10.2 MPa and 323.15 K SCCO2 can attain the desired n-3/n-6 ratio in 6 h. Because of the negative effect on the enzyme activity by the enriched n-6 TG, the optimal substrate ratio of the enriched n-6 TG and the n-3 PUFA was chosen as 1/4. To enhance the solubility of n-3 PUFA in SCCO2, ethanol was applied as a co-solvent and reached an optimal input at 10 wt.% of total substrates. The activity of the enzyme still maintained 81% of initial activity because of de-moisture from the surface enzyme after seven cyclic pressurization/depressurizations. A model of the ping-pong mechanism and the substrate inhibition was proposed to express the catalytic action of lipase. With the aid of the Michaelis-Menten equation and Lineweaver-Burk plot, the estimated reaction rate has a deviation of only 2.11% from the experiment data. (C) 2008 Elsevier B.V. All rights reserved.