Recently, a new synthetic methodology of rhodium-catalyzed carboamination/cyclopropanation from the same starting materials at different reaction conditions has been reported. It provides an efficient strategy for the stereospecific formation of both carbon- and nitrogen-based functionalities across an alkene. Herein we carried out a detailed theoretical mechanistic exploration for the reactions to elucidate the switch between carboamination and cyclopropanation as well as the origin of the chemoselectivity. Instead of the experimentally proposed RhIII-RhI-RhIII catalytic mechanism, our results reveal that the RhIII-RhV-RhIII mechanism is much more favorable in the two reactions. The chemoselectivity is attributed to a combination of electronic and steric effects in the reductive elimination step. The interactions between alkene and the rhodacycle during the alkene migration insertion control the stereoselectivity in the carboamination reactions. The present results disclose a dual role of the methanol solvent in controlling the chemoselectivity.