The present invention is directed to a process for hydroprocessing of a liquid hydrocarbon feedstock, comprising: (a) mixing liquid, partially vaporized and/or vaporized hydrocarbon feedstock with molecular hydrogen; (b) feeding said mixture into a compression reactor; (c) compressing said mixture to a pressure, a temperature and for a residence time sufficient to: i) thermally crack at least a portion of hydrocarbon molecules in said hydrocarbon feedstock, and ii) react hydrogen in the presence of a hydrogenation catalyst with unstable portions of the cracked molecules, forming a hydroprocessed product; and (d) expanding said mixture to reduce the pressure and temperature thereby reducing subsequent undesirable reactions.