In an attempt to correctly understand an extra weak exothermic peak (T_h) (near the melting temperature of the polyethylene (PE)) of the PE in the PE/boron nitride (BN) composites, a new hypothesis was proposed and proved: T_h was induced by PE crystallization. When the BN content was more than 10 wt%, the T_h was observed. Simultaneously, beside the T_h, another weak exothermic peak (T¹_h), was also found. Beside the main melting peak (T_m), an extra melting peak (T_mh) also appeared. The results of the local thermal analysis technique (nano-TA) showed that the local melting temperature (nano-T_m) of the PE near the BN aggregates was higher 4–8 °C than that in other areas, indicating that the meso-phase (meso-phase was also induced by PE crystallization) can be formed near the BN aggregates during the PE crystallization. Moreover, the appearance of the T_h and T¹_h was attributed to the differently nucleated capability of the different BN aggregates in local areas during the PE crystallization. When the annealing time and temperature were 20 min and 130 °C, respectively, the thermal conductivity of the PE/BN composite was 16% higher than that of the unannealed PE/BN composites. In addition, the results of the wide angle X-ray diffraction (WAXD) showed that the BN particles had no influence on the PE crystal form in the PE/BN composites.