The authors have been carrying out studies of burning lower grade heavy fuels.  As for “high viscosity” and “low quality”, it is learned from many studies that these two words do not always have the same meaning. In fact, the influences on diesel engine performance are different for fuels with the same viscosity level but from different refinery processes or crude oil sources.From these points of view, a series of experiments has been carried out on a medium-speed diesel engine using high viscosity fuels up to l000cSt (50°C) prepared by blending the vacuum residue and different diluents, which are marine diesel oil, light cycle oil, and FCC slurry. Each neat diluent has also been tested in order to compare it with the high viscosity fuels.The ignitability of fuel greatly affects engine performance and combustion characteristics. Moreover, combustion characteristics are varied even if viscosities are the same.Deterioration of smoke; specific fuel consumption, and deposits in the combustion chamber are high at low load. However, combustion deterioration at low load can be improved by increasing air temperature and injection pressure.When blending two kinds of fuels, the ignitability of the blended fuel can be determined according to the ignitabilities of the component fuels and their blending ratio. Ignitabilities can be well expressed by CCAI, etc. In this case.It was impossible to continue stable operation when the ignition delay exceeded about 3 msec as far as this experimental work was concerned. The fuel corresponded to a CCAI of about 860 and higher in such cases.This paper discusses the above results and focuses mainly on the engine performance and combustion by means of ignition delay, heat release rate and other factors. The injection characteristics of using heavy fuels are also discussed in the paper.