An experimental program was conducted to determine the effect of burning used lubricating oil mixed with fuel oil in a single cylinder Diesel engine. With increasing fuel costs, the prospect of using alternative fuels increases dramatically. Large quantities of used lubricating oil are currently available and could be used in fuel blends for Diesel engines.The used lubricating oil-fuel oil ratios used in this study were 2.5%, 5%, 10%, and 15% used lubricating oil by volume. The effect of burning these blends on unburned hydrocarbons, carbon monoxide and oxides of nitrogen is reported along with a comparison to baseline values for pure Diesel fuel. Tests were run at 1800, 2400 and 3000 rpm at 1/4, 1/2, 3/4 and full rack positions for each fuel blend. The general condition of wear in the engine along with deposit formation and analysis is reported. The effect of the used lubricating oil on thermal efficiency and BSFC is shown for the blends studied.