Geometrical structures of iron oxide cluster cations have been analyzed by ion mobility mass spectrometry. The series of (FeO)(n)(+) and FenOn + 1+ cluster cations were predominantly observed in a mass spectrum at high ion-injection energy into a drift cell. Arrival time distributions in the ion mobility spectrometry indicate that two structural isomers coexist for the (FeO)(n)(+) clusters at n >= 5. By comparison of experimental collision cross sections determined from the arrival times with theoretical ones, two-dimensional ring and sheet structures were assignable for (FeO)(n)(+) (n = 3-8). In addition to these isomers, compact three-dimensional structures were also found to be stable at (FeO)(n)(+) (n >= 6). Thus, the two-dimensional and three-dimensional structural isomers coexist for (FeO)(n)(+) (n = 6-8).