Microwave assisted magnetization switching (MAS) has been expected as one of the prominent future ultra-high density magnetic recording technologies. In this paper, we have studied the rf field dependence of the switching behavior of Co/Pt nanodot arrays with various dot diameters. While the critical frequency increases and the minimum switching field decreases with increasing the rf field, MAS behavior critically depends on the dot size. For small dots less than 130 nm in diameter, the critical frequency and the minimum switching field exhibit linear variation with the rf field. On the other hand, for the larger dots (dot diameter ≥ 230 nm), they exhibit not only profound MAS effect but also obvious change in the behavior at a certain rf field. These changes of MAS behaviors are explained as a consequence of the change of magnetization precessional mode. On the other hand, the dispersions of MAS effect exhibit no remarkable variation on the rf field.