The effect of reduced pressure ranging from 0.05 to 0.86 atm on oil yield was determined as a function of heating rate, the flow rate and type of sweep gas, and the grade of oil shale. A lower pressure, higher heating rate, and higher flow rate of sweep gas produced a greater amount of oil. Retorting of Colorado oil shale at 0.05 atm with a heating rate of 12 K/min produced oil yield as high as 107 wt % of Fisher Assay (FA) with nitrogen and 112 wt % of FA with water vapor. The oil yield was substantially the same with nitrogen and carbon dioxide. The Michigan oil shale, containing a higher fraction of aromatic carbons than the Green River Formation shale, produced a higher percentage of FA oil yield under a reduced pressure.