This paper presents CAE method enhancement which realizes gear noise time domain simulation to practical use for AISIN AW automatic transmission assembly model. Today, vehicle electrification has been accelerating due to severe fuel consumption regulations. Furthermore, as quietness of interior noise of vehicle is improving, further gear noise reduction of A/T is imperative. Currently, as one of the countermeasures for realizing lightweight and NV reduction, we apply elongate holes in gear flange. Although the effect of gear noise reduction is confirmed in evaluations, the mechanism for this gear noise reduction by the elongate holes has not been clarified completely and the current simulation based on modal analysis with static condition cannot replicate actual NV phenomena of A/T assy. Furthermore, it takes an enormous amount of time and cost since the design study for elongate holes is based on the evaluation results. Therefore, clarification of the mechanism and development of NV simulation enhancement which can replicate NV phenomena is required for efficient design study. As a result of evaluation for verifying NV phenomena by elongate holes, we confirmed gear noise level reduction because a resonance frequency of the gear is fluctuated owing to the stiffness difference between hole part and lib part of the gear. This time, in order to replicate the NV phenomenon with above mentioned mechanism by CAE method, we apply gear noise time domain NV simulation to practical use for A/T assy model by using “multi body simulation” based on modal synthesis method. As a consequence, we present that validity of time domain NV simulation was confirmed by the evaluation for gear noise of A/T assy.