A numerical approach to improve prediction capability of transonic buffet onset based on URANS simulation is investigated. In the present approach, small velocity perturbation is added in every computational cell to force flow instabilities to appear otherwise dissipated in URANS simulation. The computed buffeting flowfield over the NASA-CRM wing is compared with that given by DDES. It is shown that unsteady shock excursions only appear in the outboard wing region in both results for M∞ = 0.847, α = 4.65° and Remac=2.26 × 106. The computed results obtained so far are encouraging that the present unsteady perturbed RANS simulation could predict the buffet onset with reasonable accuracy in less computing time which, however, is yet to be clarified in the future study.