Polyamicacids using BTDA (3,3',4,4'-benzophenone tetracarboxylic dianhydride) with 2,4-TDA (2,4'-toluene diamine) and 4,4'-MDA (4,4'-methylene dianiline) were synthesized by condensation polymerization. Blend solutions of poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole](PBI) and the above synthesized polyamicacids were prepared by using a cosolvent, DMAc and cast film or precipitated into powder, then cured at higher temperatures than the estimated T_g's. Miscibility and the specific interactions responsible for miscibility were investigated using DSC and FT-IR. Phase diagram was obtained by DSC and thermal stability of the blends was also stooled using TGA. Both the blend systems, PBI/BTDA+2,4'-TDA(blend Ⅰ ) and PBI/BTDA+4,4'-MDA(blend Ⅱ) are miscible ; evidences for miscibility were optically clear films and single composition dependent T_g's intermediate between those of the homopolymers. LCST type phase behavior was obtained by DSC annealing study and thermal stability was enhanced in these blend systems. From FT-IR studies, carbonyl stretching band of PIs and N-H stretching band of PBI shifted to lower frequencies about 7∼10cm^-1 and 50∼55cm^-1, respectively. Thus hydrogen bonding between carbonyl arised from 5-membered ring solely and N-H is responsible to bring a miscible system.