The presence of long-chain branching provides advantageous properties to a molten polymer in film production, especially for a fast growing commodity plastic such as polyethylene. Besides the innovation of polymerization, the most attractive method to tailor the molecular structure is peroxide modification via reactive processing which is the post-reactor technique. By this process, it can achieve the enhancement of ability of the melt to be drawn or stretched during the film processing due to branching. However, to avoid the undesired excess of macroscopic molecular network, which may cause defects in the film product, a small quantity of peroxide initiator was used to modify the base resin. The rheological properties, as related to molecular characteristics, were investigated using a capillary rheometer. Rheological analysis of the products modified by different peroxide addition methods was of interest as changes in rheological properties depend markedly on the efficiency of incorporating and mixing the peroxide into the polymer. Process parameters (including mixing temperature, peroxide quantity, and mixing speed), which appear to be responsible for the molecular characteristics alteration, were considered in order to produce materials with preferred rheological properties.