The InP(l00) (4×2) reconstructed surface prepared by argon ion bombardment and post annealing under phosphorus pressure has been investigated by LEED, HREELS and UPS. It was found by HREELS that the forming of surface In-H and P-H bonds during hydrogen exposure happened almost simultaneously. By measuring the UP spectra using polarized light sources, it can be verified that the dangling bond states of surface P atoms do not possess the symmetrical characteristic with respect to the two basis vector directions of surface unit mesh. It could be deduced from the above evidences that there must exist some In vacancies on the In-rich InP (100) surface, and the dangling bonds of those P atoms which are located next to the In vacancies orientate off their normal directions. We propose here a missing row-dimer model for the atomic arrangement of InP (100) (4×2) stucture, which could explain the observed experimental qualitatively and might be used as a working model for seeking a precise surface structural model.