This study was examined the thermal behavior of graphene nanosheets/carbon nanotubes-water nanofluid using the molecular dynamics method. First, the atomic stability in simulated structures was investigated by examining kinetic and potential energies. The results of this part represent the convergence of physical quantities. Also, the simulated samples' atomic and thermal behavior was studied by examining independent variables, including the volume fraction and the dimensions of carbon nanoparticles (graphene nanosheets/carbon nanotubes). The molecular dynamics simulations show that with the addition of carbon nanoparticles (NPs) with optimal value (5%), the phase change time and the thermal conductivity of the simulated nanofluid were converged to 1.10 ns and 0.73 W/mK, respectively. Also, increasing the dimensions of carbon NPs leads to a reduction in the phase change time of the simulated structure. Numerically, by increasing the length of carbon NPs to 1 nm, the phase change time in this sample reduces to 1.02 ns? Generally, these results indicate that the thermal behavior of the water-based fluid improved with the addition of carbon NPs.