The catalytic performance of Co/MgO catalysts for the treatment of tar derived from cellulose steam gasification in a bubbling fluidized bed gasifier of 22 mm i.d. and 500 mm height was investigated by means of two different methods: hot gas cleaning in a fixed bed reactor after the fluidized bed gasifier (secondary method) and treatment inside the gasifier with catalyst as a fluidizing medium (primary method). For steam gasification without catalyst, the tar produced in the gasifier was about 12 wt % of the fed cellulose. The main components of tar were cellotriosan, cellobiosan, and levoglucosan. In the secondary method with 12 wt % Co/MgO catalyst, the tar in effluent gases from the fixed bed reactor were considerably less at 873 K: tar conversion was greater than 80%. The amounts of produced H-2 and CO2 increased; however, the amount of CO remained almost at its level without catalyst. In addition, the amount of produced gases remained almost constant during the 100 min of reaction time at 873 K. Therefore, results suggest that steam reforming of tar derived from cellulose gasification proceeds sufficiently with 12 wt % Co/MgO catalyst at 873 K. The primary method showed that tar conversion increases with the Co loading amount. The 36 wt % Co/MgO catalyst showed 84% tar conversion. High carbon conversion to gas (67%) was also attained in the case of the 36 wt % Co/MgO catalyst. The amount of produced gas remained for 2 h. Results showed that the developed Co catalyst has a great potential for tar treatment in both the primary and secondary methods.