Recently, the Shuaiba Refinery of Kuwait National petroleum Company (KNPC) conducted a test run using mixed NiMo/CoMo catalysts in one of the H-Oil reactors to verify the specifications of a CoMo catalyst. However, reduced unit performance and unstable operation were experienced. This led to the unit shut-down and process time loss. To search the causes for the different reactor behaviours, a research work was undertaken in the Petroleum Technology Department at Kuwait Institute for Scientific Research (KTSR). It had found that the hydrodynamics of the mixed NiMo/CoMo and CoMo catalysts in a cord flow ebullated-bed reactor (EBR) were very different than NiMo catalyst alone.