The mechanisms of seven reactions between keteniminium cations and olefins have been theoretically explored at BHandHLYP/6-31G* level. It is found that these seven reactions always form a relatively stable hydrogen-bonded type of ion-molecule complex first except for reactions 1d+2a and 1e+2a, which have no hydrogen atom attached to nitrogen atom in keteniminium cations. Some reactions take place via a concerted but unsynchronous mechanism, and the others are stepwise processes. The substituent effects are also studied. The data reveal that the electronpushing substituents on keteniminium cations disfavor the reaction, and the electron-attracting substituents on keteniminium cations favor the reactions. The substituent effects on ethene are contrary to the former case.