Development of heavy hydrocarbon into toluene insoluble organic residue (TIOR; or coke) is not a favored reaction pathway in an upgrading process of petrolium residue. In many commercial catalytic hydrocrackers, a small amount of coke formation can cause a serious operating problem of clogging and unexpected instability due to its continuous accumulation on the process units including reactors. In general, a high hydrogen partial presssure or a low reaction temperature below 400C, which requires a high CAPEX and/or OPEX, is favored to reduce coke formation. In the presentation we will discuss the reaction behaviors of coke formation and an effective strategy to suppress the pathway to coke during the slurry-phase hydrocracking.