This invention relates to oil refining, more particularly to a composition of a heavy fuel oil for use in marine power units and boiler plants. The fuel comprises, in percent by weight: an extract from the selective extraction of gas oil--(3-5), heavy catalytic cracking gas oil--(3-10), vacuum gas oil--(5-10), tar or topped residuum--(3-10), straight-run black oil--(10-20), viscosity breaking residue of heavy oil fractions--the rest. The selected combination of the components and their ratio may for example, result in a stable heavy fuel oil having improved viscosity properties. The inclusion of the above components into the fuel composition may also make it possible to expand the production of heavy fuel oils produced.