•Gas hold up and flow regimes is investigated under hydrocracking condition for the design of a slurry phase hydrocracking reactor for heavy oil.•VR conversion considerably affect the gas hold up and the flow regime transition.•In the heterogeneous regime, the performance of the hydrocracking reaction is reduced due to backmixing effect.•A flow regime map is proposed to facilitate the reactor design for the slurry-phase hydrocracking reactor. This study investigated gas hold up and flow regimes according to operating conditions (temperature, pressure, superficial gas velocity) for the design of a slurry phase hydrocracking reactor for heavy oil. Heating medium oil, vacuum residue (VR) and hydrocracking products were used in a bubble column reactor which had a diameter of 0.05 m and a height of 2.3 m. The results confirmed that the gas hold up increased and the flow regime transition point was delayed as VR was converted to a hydrocracking product (change in physical properties), and pressure also affected the flow regime transition. Using the dynamic gas disengagement method, the flow regime transition was observed in the actual hydrocracking reaction, and at the same time, it was confirmed that the hydrocracking reaction performance decreased. A VR slurry phase hydrocracking flow regime map is proposed by modifying the existing correlations, and a guide for reactor design and operation is suggested.