Abstract: Based on the basic data of a gas oil hydrotreating unit under various process conditions, the changing rule of liquid phase volume fraction in up-flow reactor and the effect of operation parameters on the liquid phase volume fraction are investigated. It is found that in an up-flow reactor, the liquid phase volume fraction increases along the inlet to the outlet of the catalyst bed gradually; the composition of make-up hydrogen, operation pressure and temperature only affect the liquid phase volume fraction slightly, yet recycle oil ratio and the feeding mode of make-up hydrogen have significant influence: the liquid phase volume fraction increases with the increase of recycle oil ratio and adopting multi-point feeding of make-up hydrogen can improve the liquid phase volume fraction effectively. The liquid phase volume fraction at the reactor outlet can be predicted by calculation using mathematical correlation equations, thus automated process control is made possible through DCS system to maintain stable operation of the unit.