A gas injection port structure for a flat fluorescent lap (FFL) is provided. The FFL has a flat lower plate and an upper plate that form a channel therebetween, and at least one gas injection port in communication with the channel. The gas injection port may be formed at a predetermined position on the upper plate so that a height of the gas injection port is level with or lower than a height of the channel. The gas injection port may contain a mercury getter and a sealing material having a passage formed therethrough so that mercury vapor may be injected into the channel and then the channel sealed. The gas injection port minimizes a thickness of the FFL, and improves durability of the FFL.