Demetalization of heavy oil through pyrolysis in the presence of subcritical water or supercritical water (SCW) was experimentally investigated. At a high water-to-oil ratio and high water density, the occurrence of pyrolysis can be transferred to the SCW phase. Driven by the pi-pi attractive interaction between aromatic sheets and the superb diffusivity in SCW, the coke-like self-assembly of metal-containing heavy aromatics occurs spontaneously and rapidly. The self-assembly behavior of aromatics in SCW depends not only on the thermodynamic state of SCW but also on the average scale of aromatics. With the aid of self-assembly in dense SCW, the condensation of metal-containing heavy aromatics, distributed mainly in a vacuum residue, to coke is significantly accelerated, by which the rate of demetalization is improved simultaneously. Owing to the preferential self assembly of metal-rich heavy aromatics, an increasing yield of liquid products can also be obtained under an optimized SCW environment.