A gigabit passive optical network (GPON) system for fiber to the home (FTTH) service must provide a down-stream data rate of an optical band to provide IPTV service with hundreds of channels to subscribers, and must be able to provide an upstream data rate of an optical band using a currently available BM-IC chip. A currently available BM-IC chip for a GPON has 1.244 Gbps and 2.488 Gbps modes. Accordingly, an optical network terminal (ONT) for a GPON that is capable of providing a downstream transmission band of 10-Gbps and an upstream transmission band of 1.244 Gbps or 2.488 Gbps, and a method for processing an upstream frame in the terminal, are provided. The GPON ONT can provide 20 Mbps, high-definition IPTV service with 500 channels and can provide both upstream data rates of 1.244 Gbps and 2.488 Gbps according to a user's selection without using an additional device.