A transmitted interference method for nanovolume detection is developed for a square-capillary or microchannel. A He-Ne-laser beam passed along an interface of inner medium/capillary wall or microchannel. The inner medium- and capillary wall-passed parts of the laser beam interfered each other to form an interference fringe after exiting out of the capillary. Experimental results of intensity profiles of the interference fringe coincided well with theoretical ones. The method was demonstrated to be applicable as a new universal tool for nanovolume detection.