The effect of the branch content (BC) and composition distribution (CD) of linear low-density polyethylene (LLDPE) on the thermal and mechanical properties of its blends with LDPE were studied. All blends and pure resins were conditioned in a Haake PolyDrive blender at 190 degrees C and in the presence of adequate amounts of antioxidant. Two metallocene LLDPEs (m-LLDPE) and one Ziegler-Natta (ZN) hexene LLDPE were melt blended with the same LDPE. The effect of the BC was investigated by blending two hexene m-LLDPEs of similar weight-average molecular weights and molecular weight distributions but different BCs with the same LDPE. The effect of the CD was studied by using a ZN and an m-LLDPE with similar weight-average molecular weights, BCs, and comonomer type. Low-BC m-LLDPE blends showed separate crystallization whereas cocrystallization was observed in the high-BC m-LLDPE-rich blends. However, ZN-LLDPE/LDPE blends showed separate crystallization together with a third population of cocrystals. The influence of the crystallization behavior was reflected in the mechanical properties. The BC influenced the modulus, ultimate tensile strength, and toughness. The addition of a small amount of LDPE to a low-BC m-LLDPE resulted in a major improvement in the toughness, whereas the results for the high-BC pair followed the additivity rule. ZN-LLDPE blends with LDPE blends were found to be more compatible and exhibited superior mechanical properties compared to m-LLDPE counterparts with the same weight-average molecular weight and BC. All mechanical properties of ZN-LLDPE blends follow the linear rule of mixtures. However, the CD had a stronger influence on the mechanical properties in comparison to the BC. (c) 2005 Wiley Periodicals, Inc.