High‐density polyethylene/natural rubber (HDPE/NR) blends filled thermoplastic tapioca starch was studied. In this blend system, thermoplastic starch (TPS) acted as an inert component, and the influence of TPS incorporation was studied in terms of tensile properties and crystallization kinetics. Tensile properties of the blends were affected by the addition of TPS particles, which reflects the incompatibility and lack of adhesion at the interface. The effect of TPS incorporation on the crystallization behavior of the HDPE/NR blends was also evaluated at different predetermined crystallization temperatures. The isothermal crystallization data obtained in this study were analyzed by using the Avrami equation. The Avrami exponent for HDPE/NR and HDPE/NR‐10% TPS blends varied around 2.0 and slightly decreased around 1.8 for HDPE/NR‐30% TPS, implying the nucleation process is heterogeneous and the crystal growth is 2D. J. VINYL ADDIT. TECHNOL., 22:191–199, 2016. © 2014 Society of Plastics Engineers