Abstract：While Typhoon Megi (2010) shared many common features of tropical cyclones (TCs) that crossed Luzon Island in northern Philippines, it experienced a significant inner-core size increase with little eyewall contraction during its rapid intensification (RI) phase. This is unusual since the majority of TCs experience an eyewall contraction with little or a slow inner-core size increase during RI. The inner-core size increase during RI of Typhoon Megi was simulated reasonably well using the Advanced Research Weather Research and Forecasting (ARW-WRF) model with both dynamical initialization and large-scale spectral nudging. In this paper processes responsible for the inner-core size increase of Typhoon Megi during its RI phase were analyzed based on a control simulation. It is shown that the inner-core size increase was primarily related to the binary interaction of Megi with a large-scale low-level depression in which Megi was embedded. The shearing/merging of the largescale depression with Megi and the subsequent axisymmetrization led to the strengthening of the outer circulation of Megi. Both the moist condition in the low-level depression and the binary interaction contributed to active spiral rainbands. Diabatic heating in spiral rainbands enhanced low-level inflow, which brought absolute angular momentum inward, increasing tangential wind speed outside the eyewall, thus leading to the outward expansion of tangential wind and the increase of the inner-core size of Megi.