Intramolecular ortho-C-H activation and C-N/C-O cyclizations of phenyl amidines and amides have recently been achieved under Cu catalysis. These reactions provide important examples of Cu-catalyzed functionalization of inert C-H bonds, but their mechanisms remain poorly understood. In the present study the several possible mechanisms including electrophilic aromatic substitution, concerted metalation-deprotonation (CMD), Friedel-Crafts mechanism, radical mechanism, and protoncoupled electron transfer have been theoretically examined. Cu(Ⅱ)-assisted CMD mechanism is found to be the most feasible for both C-O and C-N cyclizations. This mechanism includes three steps, i.e. CMD with Cu(Ⅱ), oxidation of the Cu(Ⅱ) intermediate, and reductive elimination from Cu(Ⅲ). Our calculations show that Cu(Ⅱ) mediates the C-H activation through an six-membered ring CMD transition state similar to that proposed for many Pd-catalyzed C-H activation reactions. It is also interesting to find that the rate-limiting steps are different for C-N and C-O cyclizations: for the former it is concerted metalation-deprotonation with Cu(Ⅱ), whereas for the latter it is reductive elimination from Cu(Ⅲ). The above conclusions are consistent with the experimental kinetic isotope effects (1.0 and 2.1 for C-O and C-N cyclizations, respectively), substituent effects, and the reactions under O2 -free conditions. Intramolecular ortho-CH activation and CN / CO cyclizations of phenyl amidines and amides have recently been achieved under Cu catalysis. These reactions provide important examples of Cu-catalyzed functionalization of inert CH bonds, but their mechanisms remain poorly understood. In the present study the Several possible mechanisms including electrophilic substitution substitution, concerted metalation-deprotonation (CMD), Friedel-Crafts mechanism, radical mechanism, and protoncoupled electron transfer have been theoretically examined. Cu (II) -assisted CMD mechanism is found to be the most feasible for both The calculations include that Cu (II) mediates the activation of Cu (II) intermediate and reductive elimination from Cu (II) an six-membered ring CMD transition state similar to that proposed for many Pd-catalyzed CH activation reactions. It is also interesting to find that the rate-limiting steps are different for CN and CO cyclizations: for the former it is concerted metalation-deprotonation with Cu (II), whereas for the latter it is reductive elimination from Cu (III). kinetic isotope effects (1.0 and 2.1 for CO and CN cyclizations, respectively), substituent effects, and the reactions under O2 -free conditions.