An apparatus for dynamically managing a group transient key (GTK) and a method thereof in order to perform setting of a GTK successfully by an access point (AP). Wherein, the AP checks security state of a plurality of mobile stations (MS)s connecting to the AP, and exchanges and sets a GTK for authenticated MSs. The apparatus for managing a GTK in a wireless LAN system, the apparatus including: a GTK generation timing deciding unit for deciding timing to generate a GTK based on security state of an MS; a GTK generating unit for generating a GTK according to the GTK generation timing decided in the GTK generation timing deciding unit; a GTK exchanging unit for exchanging the GTK generated in the GTK generating unit based on the security state of the MS; and a GTK setting unit for setting the GTK based on the number of MSs exchanged the GTK.