To prepare organo-soluble poly[(2,2,'-m-phenylene)-5,5'-bizenzimidazole] (PBI) with high yield, a homogeneous nitration of PBI was attempted. Nitro-substituted PBI (NO2-PBI) was synthesized through the homogeneous reaction of the PBI powder with nitric acid in sulfuric acid. The degree of substitution (DS) of this NO2-PBI is higher than that of the NO2-PBI prepared through the heterogeneous reaction of the PBI fiber. The viscosity of the NO2-PBI prepared through the homogeneous reaction decreased with increasing amount of nitric acid added. The DS of the NO2-PBI reached the maximum value of 2. The substitution efficiency of nitro groups decreased as the amount of nitric acid added increased. When a small quantity of nitric acid was added, the substitution of the sulfonic acid group was confirmed as well as that of the nitro group. The solubility of the NO2-PBI depended strongly on the DS. The NO2-PBI having the DS of about 2 was completely soluble in dimethylacetamide and almost soluble in N-methylpyrrolidone. At an elevated temperature, it was also soluble in other polar aprotic solvents such as dimethylformamide and dimethylsulfoxide.