A lamp has a light tube, an illuminant module mounted in the light tube and two end caps mounted respectively on two opposite ends of the light tube. A slot, being substantially inverted T-shaped in cross-section, is formed in and axially along the light tube and the end caps. A guide rail or a pair of resilient plates is disposed at a presupposed position and is mounted through the slot of the lamp to hold the lamp. An amount of lamps held on the guide rail or the pair of resilient plates is changeable according to needful brightness. Therefore, disposing the lamp is easy, quick and convenient, and a total cost for disposing the lamp is low.