A roaming method and a data transmission system for a portable terminal in a Wireless Local Area Network (WLAN) are provided. The method includes transitioning to an active mode, by a control unit in a standby mode in response to a request from a WLAN module during roaming from a first Access Point (AP) to a second AP, and performing authentication in conjunction with a server, by the control unit in the active mode. As a result, when a portable terminal roams in a WLAN, the network connection is not disrupted.