Hexagonal boron nitride (HBN) nanosheet, exfoliated by a facile Li+-assisted hydrothermal exfoliation method from pristine HBN, is first used as supporting material and thermal conductivity additive to prepare form-stabilized composite phase change material (PCM) by vacuum impregnation. Stearic acid (SA) was stabilized by three types of HBNs with different exfoliation degrees. Exfoliation results show that the final exfoliated HBN nanosheet exhibits maximum thickness around 5-7 nm and minimum thickness around 2-3 nm. The single-layer crystal structure is hardly destroyed and surface properties are almost no change in the exfoliation process. Furthermore, thermo-physical properties results indicate that the loading capacity of exfoliated HBN was enhanced by 32.02% compared to pristine HBN. The corresponding composite PCM has excellent chemical compatibility and thermal stability under 180 degrees C. Moreover, the latent heat of composite is as high as 136.20 J g(-1), and crystallinity is up to 98.57%, signifying almost SA in the framework of support can conduct thermal molecule movement and crystallization normally. It also indicates that the thermal conductivity of composite (0.453 W m(-1) K-1) is enhanced by 73.36% than pure SA (0.261 W m(-1) K-1), showing a good heat transfer efficiency. Consequently, this exfoliated HBN nanosheet possesses high loading capacity and thermal conductivity, which has the potential to prepare form-stabilized composite PCMs for thermal energy storage applications.