The photoionization efficiency (PIE) spectra of the ions produced by the VUV synchrotron radiation photoionization of (CH3I)(n) (n = 1-4) molecules in the photon energy range of 8-35 eV have been measured by using time-off-light (TOF) mass spectrometer with supersonic cooling techniques. The appearance potentials (APs) of all the observed fragment ions have been determined from the PIE curves. Based on these data, the bond energies (D-0), standard formation enthalpies (Delta (f)H(0)degrees) and proton affinities (PA(0)) of the related fragments were obtained. Several autoionization structures of CH3I in the PIE spectrum of CH3I+ were assigned.