Abstract: Structure change would occur when typhoons approach or cross an island, and that will usually give rise to their intensity change, track turns, and variations i n associated high winds and heavy rains. A phenomenon of eyewall expansion relat ed to Typhoon Sepat (0709) was found when it was crossing the Taiwan Strait. A s tudy was conducted to investigate the causes of the eyewall expansion and its re lationship with the typhoon intensity change using data from Shanghai Typhoon In stitute, FY-Ⅱ satellite remotely sensed images, Taiwan hourly radar echoes, an d NCEP 1°×1° reanalysis fields. It is found that (1) Sepat's eyewall actually broke down and dissipated after it made landfall at the island of Taiwan, but a new and expanding eyewall formed as soon as Sepat entered the Taiwan Strait an d approached China mainland. It expanded into a big eye with about 600 km in dia meter finally. (2) The eyewall expansion actually was a formation process of clo ud band ring, which was composed by the diffused convective cloud clusters from the core region of typhoon and its spiral bands, It was associated with many factors including the decrease of underlying temperature, the weakening of unst able atmospheric stratification, the enhancement of outgoing winds, and expandin g descending motion in typhoon core region, etc. (3) An arc humidity frontal zon e occurred in the western periphery of the typhoon in lower layer due to the int rusion of dry air from environmental circulation. The front was favorable for th e development of convection, but disadvantageous for the further expansion of ey ewall clouds because of the descending motion on the dry side of the frontal. In this connection, diffused convective cloud clusters were organized by the circu lar typhoon circulation into an arc form near the front and joined the spiral cl oud clusters in the eastern part of the typhoon circulation, leading to the form up of a ring eyewall. (4) The typhoon decaying was closely related to the typho on eyewall expansion. Because condensational heating was weakened and the heatin g source was dispersed over the typhoon center in this process, the typhoon warm core couldn't be sustained, resulting in the dereasing of typhoon intensity. On the other hand, the input of environmental water vapor and energy was held back by the enhancing radial outflow of the typhoon in lower to middle layers, conve ction in the typhoon core region was thus attenuated.