We have systematically investigated the ground state domain structure transition on magnetite (Fe3O4) material using micromagnetic simulation. A spherical model of magnetite nanoparticle was simulated with varied diameter size from 40 nm to 100 nm. It is observed that the domain structure transition from single domain to multi-domain occurred between 68 nm and 69 nm. The changing of domain structure is followed by changing the magnetization energy. For a single domain particle, it is observed that the demagnetization energy is dominant to exchange energy while the opposite occurs in multi-domain particle. Then, we have also calculated the critical diameter based on Brown and Kittel formulas. Comparing to the formulas, our micromagnetic results are in the range between Brown and Kittel prediction, which are 62.9 nm and 83 nm, respectively. Therefore, the observed domain structure substitution from single to multi domain structure in this study is related to the critical diameter, which the magnetization energy system changed.