The extraction characteristics of lipids from powdered sea mustard have been investigated by the use of supercritical carbon dioxide($SC-CO_2$) and cosolvents. The extraction rate was increased as the particle size of the sea mustard is smaller, the pressure is higher, the temperature is lower, and the quantities of the fluid is more. However, the extraction yield of lipids from sea mustard was almost constant at a given condition. The optimum extraction condition was determined with the extraction yield of $1.45wt\%$ at 300um of particle size, 313K of $SC-CO_2$ temperature, 13.8MPa of pressure, and 30L/min of flow rate. Ethanol was the most efficient cosolvent among ethanol, methanol, and hexane. The extraction yield was increased at about 2.21times by the addition of ethanol as a cosolvent to $SC-CO_2$. As the residence time and the average concentration of lipids were decreased, the mass transfer parameter($k_fa$) was increased. But the opposite result was obtained when the ethanol was used as cosolvent.