In order to reduce the rolling force and the roll wear, the lubricants have been used in hot sheet rolling of steel, but the lubrication behavior at the interface between roll and workpiece in hot steel rolling have never been well understood. On the other hand, hot rolling process with high reduction in thickness have been developed to produce the ultra-fine grained steels. The high reduction in hot rolling causes some troubles such as the increase of the rolling force, the occurrence of friction pick up and so on. To solve these problems, the lubrication behavior at the interface between roll and workpiece in hot steel rolling must be quantitatively understood. In this paper, the coefficients of friction were measured by using the newly developed simulation testing machine for hot rolling. The effects of lubricant factors such as the compositions and the viscosity of the base oil and the additive agents on the coefficient of friction were investigated. From these experimental results, the lubrication behavior at the interface between roll and workpiece was investigated and the lubrication mechanism was proposed.