PURPOSE: A media access control protocol operating method and an Ethernet optical network system using the same are provided to enhancing an optical media access function of an optical subscriber line section and transmission delay time by providing an optimum band width by using both a dedicated band width and a dynamically variable band width service. CONSTITUTION: An OLT(Optical Line Terminal)(11) has a transmission and reception function of terminating one end of a line, fixedly allocates a dedicated band width, a variable band width and a certain band width according to a band width service form that ONUs(Optical Network Unit)(12-14) request respectively, and provides an optimum band width to respective ONUs(12-14) by using a mixed(dedicated+variable) band width service variably supporting an extra band width in a frame. The ONUs(12-14) has a transmission and reception function of terminating one end of the line at the side of an optical subscriber and also has a function of requesting a required band width from the OLT(11). A passive optical distributor(15) distributes a signal from the OLT(11) to the ONUs(12-14) or combines signals received from the ONUs(12-14) to one signal and transmits it to the OLT(11) according to a transfer direction of an optical signal.