This article reveals the dependence of crystalline phases in titania on the intrinsic activity during in situ polymerization of ethylene/1-hexene using the zirconocene/dMMAO catalyst to produce LLDPE/TiO2 nanocomposites. First, the TiO2 nanoparticles having different crystalline phases were employed as the nanofillers by impregnation with dMMAO to obtain dMMAO/TiO2. Then, copolymerization of ethylene/1-hexene using zirconocene catalyst was performed in the presence of dMMAO/TiO2. It was found that the catalytic activity derived from the anatase TiO2 (A) was about four times higher than that obtained from the rutile TiO2 (R). This was likely due to higher intrinsic activity of the active species present on the TiO2 (A). In addition, increased [Al](dMMAO)/[Zr](cat) ratios apparently resulted in enhanced activities for both TiO2 (A) and TiO2 (R). However, the TiO2 (R) showed less deactivation upon increased [Al](dMMAO)/[Zr](cat) ratios. This can be attributed to strong interaction between dMMAO and TiO2 (R) as proven by the TGA measurement. The microstructure of the LLDPE/TiO2 obtained was found to be random copolymer for both TiO2 (A) and TiO2 (R).