A high density polyethylene resin (HDPE) was modified by promoting long chain branching (LCB) through a novel photo-initiated reactive extrusion (REX) process. Surface response methodology based on a central composite experimental design was employed with three processing variables, namely, photo-initiator concentration, polymer throughput, and extruder screw speed. Samples were characterized using linear viscoelastic (LVE) measurements, shear creep tests, gel content analysis, and gel permeation chromatography (GPC). Gel content analysis showed no gel formation under the conditions used. Linear viscoelastic measurements showed an approximate tenfold increase in zero shear viscosity and hundred times increase in average relaxation time. GPC analysis showed addition of LCB up to 0.055 branches per 1000 monomer units, a slight increase in molecular weight and no significant change in polydispersity.