In this short communication, vacuum residue, bitumen, and pyrolized fuel oil were analyzed using matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) analysis to investigate the molecular weight distribution (MWD) variation of heavy oil during visbreaking. The visbreaking of vacuum residue was carried out at 350 degrees C, 400 degrees C and 450 degrees C. Vacuum residue products were studied based on MWD obtained by MALDI-TOF analysis. The separation of MWD was carried out in 8 sections, and each area of the range was calculated. According to our study, the cracking reaction dominated, even though condensation and polymerization were carried out simultaneously. It was noted that the relatively high molecular ranges of 3 and 4 were shifted to the light molecular range of 2. The MWD separation method obtained by MALDI-TOF is a potential analytical tool for investigating the MWD variation of heavy oil. (C) 2016 Elsevier Ltd. All rights reserved.