With the advent of CLSM in the end of 1980s, it has been applied to the field of pulp and paper science in various ways. This study showed the potentials of CLSM In analyzing a change of pulp fiber and paper properties before and after mechanical treatment. In particular, a quantification of internal fibrillation has been done using cross-sectional images of fibers and image analysis technique, then evaluated the effects of fiber wall delamination on fiber and paper properties. It showed that the delaminated fibers were closely associated to development of the interfiber bonding in a fiber network. The CLSM made it possible to investigate a density profile along the sheet thickness, which was created by some papermaking processes like pressing, drying and calendering. Through the attempt to observe the forming procedure of a fiber network during handsheet making, the CLSM images showed that the pressing stage was considered greatly to contribute to generation of interfiber bonding with removing a free water and partly a bound water between fibers. In addition, the CLSM could be used to illustrate not only a surface profile of paper showing the extent of smoothness or roughness, but also a density profile in a B-direction of the network. Finally it became evident that the CLSM could be used as an excellent tool to predict development in fiber and paper properties before and after mechanical treatment during papermaking processes.