Abstract: Chemical fundamentals and effectiveness of typical industrial hydrogen donors such as FCC decant oil and its subfractions during mild thermal cracking of a vacuum residue were revealed. Hydrogen donor abilities of the industrial donors were first analyzed, and the thermal cracking of a vacuum residue (VR) was then studied in an autoclave by blending with a hydrogen donor—a FCC decant oil (DO) or one of its subfractions (DO3). It is shown that while the addition of hydrogen donor increases, the yield of 180℃－ fraction decreases and the yields of both 180℃～350℃ fraction and of 350℃－ fraction increase. The viscosity of product residual fuel oil at 100℃ is reduced and its stability measured by spot test is improved with the addition of hydrogen donor. When hydrogen donor addition into feedstock is over 15%, the stability of residual fuel oil is qualified for marketing. It was found that it is the relative high hydrogen donor ability of industrial hydrogen donor employed that improves the products in both the stability of residual fuel oil and the yield of light fractions when vacuum residue is mildly thermal cracked in the presence of hydrogen donors. No correlation in this process exists between residual fuel oil′s group composition and stability. Further investigation through solubility / structural analysis revealed the chemical basis for the improved stability of residual fuel oil, i.e. addition of industrial hydrogen donor can effectively inhibit both preasphaltene formation and the deterioration of association index of the asphaltene when vacuum residue is processed by mild thermal cracking.