The effect of fiber orientation on the rheological properties of short glass fiber-reinforced composites was investigated by dynamic oscillatory shearing with parallel plate fixtures. As an oscillatory shear amplitude and frequency applied to fiber-reinforced composites increased, more fibers in the composites were aligned in the flow direction, thus the complex viscosity gradually decreased. This phenomenon was confirmed by observing the fiber orientations using optical photographs. The complex viscosity depended upon the strain amplitude, and pre-oscillatory shearing frequency, and shearing time. Experimental results for fiber orientations and complex viscosity were compared with predictions available at the present time. The predictions of the dependence of fiber orientation upon strain amplitudes and fiber volume fractions are in qualitative agreement with experimental data. However, there were effects of the magnitude of frequency and oscillatory shearing time on fiber orientation, thus complex viscosity could not be predicted successfully although these effects were clearly demonstrated by the experiment.