The aim of this work was to obtain a polypropylene (PP) with strain hardening in elongational flow by a reactive extrusion process. This rheological behavior imposes a structural modification of the PP : linear chain to long side branches. Reactive extrusion allows the modification of PP by radical chemistry, but this modification is accompanied by significant degradation of chains : beta-scission. A polyfunctional monomer (TMPTA) associated with addition of polyethylene (PE) allows us to limit this degradation and to build a connected structure. Gel permeation chromatography measurements confirm the presence of high-molecular-weight species. With thermal analysis of modified polymers, we show the creation of copolymer PP/PE at the interface of the PE inclusions. Under these conditions, all the reactive blends present strain hardening. The elongational viscosity of all blends can be described by the Leonov model calculated with viscoelastic data. This model gives a good description of the viscosity without adjusted parameters.