In this paper, a new method of initial localization for a mobile robot only employing a laser range finder is proposed. We accomplish this by the matching of Complete Line Segments (CLS) derived from current scan of range finder and the priori-map. The definition of Complete Line Segments (CLS) and Sub-Complete Line Segments (SLS) as well as their properties and decision rules are given. We confirm that, in the angle of theory, the initial localization can be accomplished even if only one pair of CLS can be matched exactly since the CLS represent some basic but complete features of the environment. So compare with other ways, this method has simpler principle and can easily be implemented with less computational costs. The whole process of initial localization for a mobile robot is described in detail. At last promising real-world experiments involving the initial localization modules on our ATRV2 mobile robot are described.