Low-coherence optical tomography (OCT) is a novel technique with high resolution for rapid, noninvasive imaging in living biological tissues. With this technique, a thin "optical section" within a thick biological specimen can be obtained. In this paper, basic principle of OCT is described. The emphasis is on the light source which is one of the most important parts in OCT system. OCT system needs wide-spectrum (in the other words, low-coherence) light source, the obtainable high resolution is directly related to the choice of light source. Appropriate light source can satisfy requirement of OCT system, otherwise inappropriate light source will degrade the resolution of OCT image. The relationship between the high resolution and the light source is studied analytically, four kinds of light source, such as LED, SLD, femtosecond laser and fluorescent light source are described respectively. The choice of light source for OCT system is analyzed in details. The research of this paper is not only the application of laser diode, but also very important in deepen and widen the field of OCT study.