Melt flow index (MI) dependency of low-density polyethylene (LDPE) on the thermal, mechanical and rheological properties in the blends with linear low density polyethylene (1-octene comonomer) (LLDPE) has been investigated by means of differential scanning calorimeter, dynamic mechanical thermal analyzer, capillary rheometer, and rheotons. Two discrete melting and crystallizing peaks were observed regardless of the MI of LDPE in the crystalline phase. Mechanical properties of the blends were fairly dependent on the MI of LDPE. Complex shear viscosities of the blends were dictated by the LLDPE indicating that severe processing conditions may be necessary. Melt viscosity analyzed in terms of the Cole-Cole plot and the log additive rule resulted that the miscibility was improved at high shear rate and that the MI of LDPE does not affect the miscibility. However synergistic effect in the melt strength was observed for the entire blends systems used in this study. In particular, the blends containing 40∼60% LDPE are suggested to design for highly improved melt strength, processibility, and mechanical properties.