The elongational viscosity of polystyrene-potassium titanate whisker composite melt was estimated by using a Meissner-type uniaxial elongational rheometer. The time dependence of flow-induced whisker orientation in a uniaxial elongational flow field was analysed in terms of the average polar angle with respect to the flow direction by wide-angle X-ray diffraction, in order to study the influence of whisker orientation on the elongational viscosity of composite melts. The whiskers prevented the strain-hardening phenomenon in elongational flow for polystyrene. The elongational viscosity was almost independent of the orientational change of the whiskers. The orientation of the whiskers was dependent on strain only. To understand this phenomenon, whisker orientation was discussed in comparison with Jeffery theory. The tendencies of the average polar angle were similar to the tendency of Jeffery theory in a uniaxial elongational flow field.