The invention relates to oil refining, more particularly to a composition of a light fuel oil for use in medium-speed diesel engines of marine power units. The inventive light fuel oil comprises, in % by wt.: black oil--(4-10), tar--(2-10), light catalytic cracking gas oil--(10-30), heavy catalytic cracking gas oil--(5-10), a straight-run diesel fuel--(8-10), a viscosity breaking residuum of a heavy oil fraction--the rest. The selected component ratio will enable to improve the fuel stability in transportation and storage, expand the range of available fuels and increase the production volumes of quality fuels, while rationally using poor-quality products readily available at refineries.