Structural assignments of gas phase chromium oxide cluster anions, CrmOn- (m = 1-7), have been achieved by comparison between experimental collision cross sections measured by ion mobility mass spectrometry and theoretical collision cross sections of optimized structures by quantum chemical calculations. In the mass spectrum, significant magic behavior between the numbers m and n was not observed for CrmOn-, while wide ranges of compositions were observed around CrmO2m+2- to (CrO3)m- as reported previously. The (CrO3)m- (m = 3-7) ions were assigned to have monocyclic-ring structures for m = 3-5 and bicyclic rings for m = 6 and 7. In addition, gradual structural change from these cyclic structures of (CrO3)m- to three-dimensional structures of CrmO2m+2- was found for m = 4-7. The energy levels of molecular orbitals of a calculated monocyclic structure of Cr5O15- were also found to be consistent with previous results of photoelectron spectroscopy, although those of the bicyclic isomer exhibited a different behavior. Moreover, the observation of abundant ions generated by collision induced dissociations at the inlet of the ion drift cell indicates that the larger sized (CrO3)m- (m > 5) series were unstable and easily dissociated to smaller ions.