To clarify the relationship between a molecular deformation mechanism and a high Young's modulus of poly-p-phenylenebenzobisoxazole (PBO), Raman spectra were measured for fibers subjected to a tensile stress along the chain axis. The stress-induced frequency shift of the observed Raman bands could be reproduced reasonably by the normal-mode calculation under a quasi-harmonic approximation. The frequency position at zero stress and the shift factor of Raman bands were predicted for a PBO chain that agreed with the actually evaluated values. On the basis of these analyses, the molecular deformation mechanism of the PBO chain has been discussed in detail. The crystalline modulus of the PBO chain was calculated theoretically to be 458 GPa, in good agreement with the X-ray observed value of 460 GPa.