Co-gasification of pulp sludge mixed with black liquor has been suggested as a new technology that can be used in addition to recovery boiler in the chemical recovery process due to its possible superior product distribution and energy efficiency. In order to study the feasibility of this process, co-gasification of these mixed wastes was carried out in a fixed bed reactor. The operating parameters including temperature of 700, 800 and 900°C, equivalence ratio (ER) of 0.2, 0.4 and 0.6 and ratio of pulp sludge to black liquor of 50:50 and 40:60 at feeding rate of 5 g/min were examined. In addition, Ni-La-Mg/Al2O3 catalyst was prepared by co-impregnation method and mixed with raw material before feeding into system to study its effect on thermochemical conversion reaction. Major gaseous products such as CO, CO2, H2 and CH4 were analyzed by TCD-NDIR analyzer. Char and tar residue were collected and calculated by measured weight. The results of non-catalytic cases indicated that the optimal condition was temperature of 900°C, ER of 0.2 and raw material ratio 40:60 with gas yield of 69.18%, H2 to CO ratio of 0.96, lower heating value of 5.91 MJ/m3 and cold gas efficiency of 79.55%. For catalytic cases, it was found that the catalyst can improve the percentage of gas yield around 10 wt%. Lower heating value and cold gas efficiency of catalytic cases were 3.78-6.73 MJ/m3 and 46.02-90.13% while those obtained from non catalytic trials were 3.37-5.91 MJ/m3 and 34.63-79.55%, respectively. From these results, co-gasification process can be considered as alternative option for conversion pulp sludge mixed with black liquor to useful fuel products.