As one of the important components of machine tools, the quality of guide rail is a direct factor which has influence to machine tools machining precision, movement efficiency and stability. Professionalization of machine tool manufacturing and modularity of machine tool structure constantly promote the development of machine tool guide rails independence and unitization. The process of forming independent unites of linear guide in machine tool separating from machine tool itself is introduced. Three kinds of representative linear guide units were listed, and the structural feature and application situation were introduced respectively. The research information about these various units is summarized, which provides an outlook about the future development of linear guide units in machine tools.