Slurry phase hydrocracking of vacuum residue at various catalyst concentrations was performed in a semi-batch reactor to investigate the effect of catalyst concentration on the reaction performance. The secondary cracking reaction was dominant at high-residue conversion (>60 wt%) without the catalyst but it was suppressed by the catalyst. Moreover, the conversion of asphaltenes at high-residue conversions was found to be dependent on the catalyst concentration. At low catalyst concentrations (0-100 ppm), asphaltenes were converted to light products at low-residue conversions and to coke at high-residue conversions. At high catalyst concentrations (100-500 ppm), asphaltenes were mainly converted to light products even at high conversions. Based on the experimental results, equations for the onset of coke formation with respect to the catalyst concentration were proposed. The higher the catalyst concentration, the longer was the coke induction period. A kinetic model was also proposed, and the experimental data could be predicted well using the same. The transformation of residue to coke was dominant in the non-catalytic hydrocracking and the coke formation rate significantly decreased as the catalyst concentration increased.