Stangeland's kinetic model for predicting hydrocracker yields was modified to explicitly account for the mass balance closure in each individual hydrocracking reaction and for the effect of hydrogen partial pressure on the hydrocracking reaction rate. This improved kinetic model has two additional parameters. The model was applied to the modeling of a fixed bed reactor for mild hydrocracking of vacuum gas oils in the framework of the Aspen Plus(R) process simulator. The kinetic parameters were regressed using the algorithm of Levenberg-Marquardt such that the mass balance in each individual hydrocracking reaction is satisfied. The reactor model assumed gas-liquid equilibrium and was used to quantify the effect upon conversion of the operating variables-liquid hourly spatial velocity, reactor pressure, and hydrogen-to-feed ratio-showing that both the kinetic and reactor models predict the appropriate trends compared to the reference data. The practical implications of explicitly introducing the mass balance closure for each hydrocracking reaction in the kinetic model are discussed.