The present work aims at developing a heat transfer model for phase change material nanocomposite (PCMNC)-based finned heat sink to study its heat rejection potential. The proposed model is developed in line with the binary alloy formulation for smaller size nanoparticles. The present study gives a more insight into the nanoparticle distribution while the nanocomposite is undergoing phase change. The nanocomposite is placed in the gap between the fins in a finned heat sink where solidification occurs from the top and lateral sides of fins. The proposed numerical model is based on finite volume method. Fully implicit scheme is used to discretize the transient terms in the governing transport equations. Natural convection in the molten nanocomposite is simulated using the semi-implicit-pressure-linked–equations-revised (SIMPLER) algorithm. Nanoparticle transport is coupled with the energy equation via Brownian and thermophoretic diffusion. Enthalpy porosity approach is used to model the phase change of PCMNC. Scheil rule is used to compute the nanoparticle concentration in the mixture consisting of solid and liquid PCMNC. All the finite volume discrete algebraic equations are solved using the line-by-line tridiagonal-matrix-algorithm with multiple sweeping from all possible directions. The proposed numerical model is validated with the existing analytical and numerical models. A comparison in thermal performance is made between the heat sink with homogeneous nanocomposite and with nonhomogeneous nanocomposite. Finally, the effect of spherical nanoparticles and platelet nanoparticles to the solidification behavior is compared.