The degree of reduction in whisker length and whisker-surface damage is higher in metallic materials than in plastic materials. Then the special methods have been devised for making whisker reinforced metallic composite materials. Among those methods, there is the fluid mechanical alignment of whiskers developed by Calow and Wakelin.The purpose of our work is to resolve the problems of Calow and Wakelin’s method and then to obtain the optimum processing condition. This report presents the experimental results on the effects of aspect ratio of the rectangular duct cross-section on the discontinuous fiber orientation in high viscous liquid flow when the ratio of the discontinuous fiber length to the duct depth (the minor width) was comparatively large. Here the aspect ratio of the duct cross-section is defined as the width to depth ratio.As a result, it can be said that the discontinuous fibers in the duct become disarranged in the flow direction with increasing aspect ratio of the duct cross-section.