We have employed complete active space self-consistent field (CASSCF) and its second-order perturbation (MS-CASPT2) methods to study the S1 and T1 excited-state ring-opening mechanisms and S1 excited-state deactivation channels of cyclopropanone, cyclobutanone, cyclopentanone, and cyclohexanone. On the basis of optimized minima, transition states, conical intersections, refined energies, and relaxed two-dimensional S1 and T1 potential energy surfaces, we find that, with the ring-strain decrease from cyclopropanone to cyclohexanone, (1) the ring-opening S1 and T1 barrier increases from 0.0 and 0.0 to 19.7 and 10.4 kcal/mol, respectively; (2) the electronic state responsible for the dominant ring-opening reaction varies from the S1 state of cyclopropanone to the T1 state of cyclopentanone and cyclohexanone; and (3) the S1 ring opening gradually becomes inefficient even blocked in cyclopentanone and cyclohexanone. This work shows that these dissimilar excited-state dynamics could originate from different ring strain of small cyclic ketones.