The influence of high energy ball milling process, HEBM, and the presence of TiO2 nanoparticles on the non-isothermal crystallization and fusion behavior of the HDPE were investigated. HEBM was used to homogeneously disperse TiO2 nanoparticles into a high density polyethylene, HDPE. Differential scanning calorimetry was used to analyze their non-isothermal crystallization and fusion behavior while, with X-ray diffraction the crystalline structures were determined. Atomic force microscopy was used to study the influence of the presence of nanoparticles on the final morphology of the polymer. It has been demonstrated that HEBM is a good method to prepare nanocomposites of well dispersed TiO2 nanoparticles within an HDPE matrix. When nanoparticles are absent the HEBM induces reduction of crystallinity of the polymer although a double crystallization process was observed; however, when nanoparticles are present, in addition of being favored the appearance of a metastable monoclinic phase, the fraction of crystals increases as milling time increases. AFM clearly showed how well dispersed were the TiO2 nanoparticles within the HDPE and how they are localized exactly between the lamellas. This result is the first clear visual evidence confirming that well dispersed nanoparticles actually do not act as nucleating agents in semicrystalline polymers. It was also shown that a 2% by weight of well dispersed TiO2 nanoparticles within the HDPE matrix induces a more homogeneous crystallization leading to denser spherulites with thicker lamellae.