The elongational viscosity of acrylonitrile-butadiene-styrene (ABS) polymer melts containing soft or hard butadiene particles has been investigated by using a Meissner-type uniaxial elongational rheometer. The influence of the hardness of the butadiene particles on the elongational viscosity is discussed for a series of ABS polymers having similar butadiene sizes and similar degrees of butadiene dispersion. The effect of the hardness of the butadien particles clearly appeared in the strain-hardening elongational property. At a butadiene content of 20 wt.%, harder butadiene particles made the strain-hardening property weaker. At a butadiene content of 40 wt.%, ABS with soft butadiene particles exhibited almost no strain hardening and ABS with hard butadiene particles gave strain softening. Observation by transmission electron microscopy showed that the soft butadiene particles were largely stretched along the elongational direction, but almost no deformation was observed for the hard butadiene particles. The results suggested that the strain hardening or softening of ABS polymer melts is strongly correlated with the degree of butadiene particle deformation.