Uniaxial and biaxial elongational flow behavior of low density polyethylene (LDPE)/polystyrene (PS) blends (LDPE/PS=2/1) with and without 5 wt. % styrene‐ethylene‐propylene block copolymer (SEP) was studied. In these blends, LDPE was the matrix phase and PS was the dispersed phase. Measurements of the uniaxial elongational viscosity η+(t,ε̇) and the biaxial elongational viscosity ηβ+(t,ε̇) were made using a Meissner‐type uniaxial elongational rheometer and a lubricated‐squeezing‐type equibiaxial rheometer, respectively. In the linear region, the relation between the transient shear viscosity η+(t) and the transient elongational viscosity, which were predicted by three‐dimensional linear Maxwell model, i.e.,EE+(t)=3η+(t) and ηB+(t)=6η+(t), did not hold well. This may be attributed to a volumetric resistance against the deformation perpendicular to the elongational direction because of the existence of PS particles. In the long‐time region, a difference of ηE+(t,ε̇) between blends with and without SEP was observed. In the case of uniaxial elongational flow, it may be presumed that a contribution of the deformation of PS domains to the elongational flow behavior has to be taken into account when the elongational strain is large.