For the first time, a one-step (self-assembly) method is introduced to prepare a phase change material (PCM) composite consisting of palmitic acid (PA) and minimal amount (0.6%) of oleylamine-functionalized reduced graphene oxide (OA-rGO) as a supporting material. Graphene oxide was reduced and functionalized with long chain alkylamine, oleylamine (OA), to adsorb palmitic acid (PA) and was simultaneously self-assembled into a three-dimensional structure. This technique eliminates common freeze-drying and impregnation steps since the phase change material is adsorbed in situ, and there was clearly no shrinkage during the drying process. In addition, the connected graphene network provides nuclei for the heterogeneous nucleation and crystallization of PA with an enhanced heat transfer to and from the PCM and retains excellent shape-stable property which prevents the leakage of molten PA. The obtained composites exhibit a large phase change enthalpy (99.6%), enhanced thermal conductivity (150%), excellent cycling performance, and substantial sunlight adsorption.