In this work, the morphology and crystallization behavior of polyamide 6 (PA6)/maleic anhydride-grafted-ethylene–propylene–diene rubber (MAH-g-EPDM)/high-density polyethylene (HDPE) blends with different ratios of the components were studied. In order to study the influence of both thermodynamics and kinetic factors, the interfacial tension between various polymer pairs was taken into account and two processing methods were used. Contact angle measurements and rheological relaxation time simulation were used to calculate the interfacial tension. It was found that because of the reaction between PA6 and MAH-g-EPDM, the value of interfacial tension of PA6/MAH-g-EPDM binary blends calculated by the contact angle measurement was not correct and the real value α = 0.97 mN m^–1 was calculated by rheological relaxation time simulation. The two processing methods (one-step and two-step processing methods) led to different crystallization behaviors with different morphologies of the blends.