Poly(p-phenylenebenzobisoxazole) (PPBO) is a polymer having the most excellent thermal properties among the organic polymers reported to date, and the design and the preparation of PPBO with the desired shape and form are expected to provide novel PPBO-based devices and materials in the fields of automobiles, aerospace, microelectronics, etc. We here describe the fabrication and characterization of PPBO films via the solution-casting and successive thermal conversion of its precursor, alkyl-substituted hydroxypolyamides (poly(o-alkoxyphenylamide)s) that were synthesized by a low-temperature polycondensation reaction of dialkoxydiaminobenzene with terephthaloyl chloride in N-methylpyrrolidone. The formation mechanism of the thermal conversion was carefully investigated based on the analyses of the results using TGA, IR, pyrolysis gas chromatography mass spectrometry (Py-GC/MS), and evolved gas analysis GC/MS (EGA-GC/MS) measurements. A transparent PPBO precursor film provided a PPBO film after thermal treatment at 250 degrees C for 6 h and then at 300 degrees C for 6 h under vacuum. The obtained PPBO film showed excellent mechanical properties (tensile strength = 76 MPa, Young's modulus = 2.7 GPa) as well as a high thermal stability (temperature of 5 wt % loss = 670 degrees C). Such outstanding mechanical and thermal properties would significantly contribute to fulfill the requirements for next generation extreme materials/devices.