A central composite design was applied to the extraction of bio-oils from flax seeds under supercritical ethanol (Sc-EtOH) conditions. The effects of three factors (temperature, time, and biomass concentration) on bio-oil and biomass conversion yields were examined. Extraction temperature was statistically found to be the most significant factor which affected both bio-oil yield and biomass conversion. The predicted results matched the experimental results with the following coefficient of determination: (R (2)) 0.95 for bio-oil yield and 0.92 for biomass conversion yield. The composition of bio-oils consisted mainly of fatty acid ester derivatives, and other oxygenated hydrocarbons. Based on the relative concentrations of the compounds, the major compound was ethyl oleate.