Cellulose nanofiber (CNF), which was developed by Saito and Isogai (2004), are biomass materials obtained from wood pulp fiber (WPF). The CNFs, which bundle together to form a WPF, are chemically fibrillated. A single WPF has a diameter of about 20 um, but a single CNF is in 3 to 4 nm. It is this extremely small diameter that exhibits characteristic physical properties of CNF. Using such properties, CNFs hava a number of engineering applications such as nano-mesh filter, foods and fillers for composite materials. CNF suspension, made from CNF being dispersed in liquid such as water, is known to show characteristic rheological properties arising from entanglement of fibers, and it produces a three-dimensional network structure. In the present work, fiber orientation of CNF suspension has been measured when it flows through a channel under assumption of CNFs being used as fillers for injection molding of nanocompsite materials. The CNF suspension used in our experiments had a weight fraction of 2 %. The diameter of CNF, which is 3 to 4 nm, is very small compared to the wave length of visible light, thus the fiber suspension is transparent. This optical property has contributed to successfully measure the birefringence of the material, which is closely related to the fiber orientation. It has been measured with a laser when the suspension flowed through a straight channel that has a rectangular cross section. Reference Saito, T. and Isogai, A., “TEMPO-Mediated Oxidation of Native Cellulose. The Effect of Oxidation Conditions on Chemical and Crystal Structures of the Water-Insoluble Fractions”, Biomacromolecules, 5, 1983-1989 (2004) Keywords: Cellulose nanofiber, Suspension flow, Birefringence, Optical measurement