Dynamic viscoelasticity and biaxial extensional flow behavior of polystyrene melts with various molecular weight distributions have been studied in wide ranges of temperature and strain rate ε̇B. Two patterns of time dependence of the biaxial stress growth coefficient η+Bare observed with increasing reduced strain rate ε̇Bτ0, where τ0is a characteristic relaxation time. At short times, η+Bcoincides with the linear viscoelastic curve; then it shows strain hardening after that, or strain softening followed by a quasisteady state. These behaviors are observed for polydisperse polystyrenes at much higher ε̇Bτ0than those for monodisperse polystyrenes. The quasisteady‐state viscosity ηB,qsdecreases with the strain rate. Addition of a very high molecular weight component causes considerable effects on τ0and on the strain rate dependences of shear ‖η*‖ and biaxial ηB,qsviscosities as well as on the stress growth coefficient η+B, where ‖η*‖ is the absolute value of the complex viscosity. The relaxation time τ0increases very much, and the strain rate dependences of ‖η*‖ and ηB,qsbecome much weaker. The upturn tendency of η+Bbecomes prominent when compared at the same reduced strain rate ε̇Bτ0. On the other hand, when compared at the same value of the reduced viscosity ηB,qs/6η0, a similar degree of strain softening in η+Bis observed for all polydisperse samples, where η0is the zero shear viscosity.