Atomistic polybenzoxazine (PBO) thin films of thickness similar to 28 Angstrom were constructed in a vacuum from the bulk amorphous PBO by increasing one edge (the z-axis) of the cubic cell enough, 100 Angstrom, so that the atoms cannot interact with periodic images in this direction. The density profile with respect to the distance from the center of mass was obtained. The density drops rapidly at the distance of similar to 7 Angstrom from the surface, and the density in the interior (1.05 g/cm(3)) is slightly lower than the bulk density (1.1 g/cm(3)). The average location of chain ends is similar to 5 Angstrom from the surface. In the composition distribution profile, polar atoms are rich in the interior of the film while the nonpolar atoms are slightly enriched near the surfaces. The fraction of hydrogen bonds in the thin film was found to be slightly higher than that in the bulk. The predicted surface tension from the internal energy contribution is 46 +/- 6 dyn/cm.