This work focuses on the study of effect of hexagonal boron nitride (BN) on the self-orientation, rheology and thermal conductivity of thermotropic liquid crystalline polymer (LCP), which possesses a semirigid-rod molecular structure. The LCP/BN composites are prepared by extrusion and injection molding processes. The 2D-WAXD analysis evidences the presence of self-orientation and typical “skin-core” structure of LCP and LCP composites. As a consequence, there are abundant macro fibrils in the matrices. The role of the lateral dimension of BN on the properties of LCP was investigated. It is confirmed that BN with a larger lateral size can more effectively improve the TC and thermal stability of LCP. Being filled with 30% BN with a large lateral size of 50 μm, the thermal conductivity of LCP/BN50-30 composite is increased by 88.11% compared with pure LCP, in contrast, the increment ratio of 38.43% and 43.93% when the LCP is filled with BN with lateral size of 0.5 μm and 5.0 μm at the same loading.