It is very difficult to select the single longitudined mode in a Q-switched ruby laser. If two longitudinal mode are oscillated simultaneously in a laser cavity, inteference fringes woued appear on the reconctrueted image even though in making a hologram at a single exposure. These hologram interference fringes present the periodicity of coherence lenlth of the laser. Certain procedures can be adopted to acvoid the situation and ensuring single longitudinal mode operation. Hence all the interference fringe will be disappeared. The coherence of the laser is also discussed in this paper.