Abstract: Geometrical optimization and theoretical calculation of the vibrational spectra of pxylene（pC8H10）and its cation were performed using the 631G basis sets of the Gaussian98 program. The primary peaks of spectra of the first excited state are verified and the results are in good agreement with the experimental values. The peaks of spectra of ionic state are assigned using ab initio calculation, and the vibrational mode on 341 cm－1 is 3.

Key words: Ab initio, Vibrational frequency, p-xylene