To enhance the melt strength of a conventional linear polypropylene (L-PP), blends with a long-chain branched polypropylene (LCB-PP) were produced by adding 2, 5, 10, 25, 50, and 75 wt% of LCB-PP to L- PP and mixing in a twin screw extruder. It was found that, already, an addition of 10% or less of LCB-PP to L- PP leads to significant strain hardening. Elongational viscosity data of L- PP and LCB-PP and those of their blends were analyzed by the use of the molecular stress function (MSF) theory. While L- PP is characterized by the MSF parameter, beta=1 ( typical for linear melts), and shows very little chain stretch (f(max)(2) = 1: 5), melt elongational behavior of LCB-PP is characterized by the MSF parameters, beta=2 ( typical of LCB melts), and f(max)(2) = 225 ( which corresponds to a maximum stretch of molecular chains by a factor of 15). By extruding LCB-PP, a refining effect is observed similar to the refining effects seen in low density polyethylene ( LDPE), which reduces the steady-state elongational viscosity and reduces f(max)(2) to 121. A second-order mixing rule for the fractional relaxation moduli, g(i), was found to show good agreement with the linear-viscoelastic data of the blends. To simulate the elongational viscosities of the L-PP/LCB-PP blends, a similar second-order mixing rule was used for the MSF parameter, beta, while a first-order mixing rule was found to be appropriate for f(max)(2): This allows for a quantitative prediction of the time-dependent elongational viscosities of all L-PP/LCB-PP blends on the basis of the linear and nonlinear parameters of the mixing components L-PP and LCB-PP only. Comparison between the steady-state elongational viscosities as obtained from creep experiments shows good agreement with predictions.