Swine manure was treated by a high-pressure reactor for bio-oil production over a temperature range 260-360 degrees C and a retention time of 5-90 min. The effect of temperature, retention time and crude glycerol addition on the oil yield and quality of liquefaction process of swine manure was investigated. Temperature showed important effects both on oil yield and oil quality. Increasing temperature from 260 degrees C to 340 degrees C was found to increase the oil yield, carbon content, and heating value of the oil. Retention time affected the completeness of the liquefaction process. Extending the retention time was found to have more influence on the quantity rather than the quality of oil product. The highest oil yield was obtained at 24% on a dry matter basis at reaction temperature of 340 degrees C and retention time of 15 min. At the optimum conditions, crude glycerol was added in the swine manure slurry at different ratios in order to evaluate the use of glycerol as a co-substrate, to improve the bio-oil production during the liquefaction process. The oil yield increased dramatically by adding crude glycerol as the co-substrate at different manure to crude glycerol ratio. Specifically, by adding three parts of crude glycerol into one part of swine manure, the oil yield increased to 68% (g/g dry matter). However, increasing the crude glycerol addition negatively affected both the carbon content and heating value of oil. (C) 2010 Elsevier Ltd. All rights reserved.