A method for guaranteeing Quality of Service (QoS) using bandwidth reservation in a switch includes: establishing a MAC table including MAC addresses of terminals connected to each port of the switch and an identifier indicating whether or not the corresponding terminal is a VoIP terminal; calculating a VoIP bandwidth necessary for each port based on information of the MAC table, calculating a ratio of an entire bandwidth to a bandwidth equal to the entire bandwidth minus the VoIP bandwidth, and setting the ratio as a port ratio of the switch; determining whether or not the corresponding packet is a VoIP packet by filtering the MAC address of the corresponding packet upon an arbitrary packet of the switch being input; and passing the corresponding packet upon a determination that the corresponding packet is a VoIP packet, and passing the corresponding packet only when the bandwidth of the corresponding packet is within a permissible limit of the set port ratio by performing port ratio control upon a determination that the corresponding packet is not a VoIP packet.