Olivine is the main constituent of the upper mantle, and its phase transformation affects the rheology of the subduction zone. It is crucial to reveal the kinetics of olivine (α)-spinel (γ) phase transformation under differential stress. To investigate the effect of microstructural properties on phase transformations such as grain boundary energy and plastic strain, we conducted a phase-field simulation using germanate olivine, an analog of silicate olivine. We conducted the simulations under various confining pressures of 1-5 GPa, temperatures of 1000 and 1200 K, with/without plastic strain, and various grain boundary energy. Under static conditions, the volume fraction of the γ phase increases as the overpressure increases because the chemical-free energy promoting grain growth dominates over the elastic strain energy, inhibiting grain growth. Under differential stress, at a slight overpressure, the volume fraction of the γ phase increases proportion to the chemical-free energy’s magnitude. Meanwhile, at a significant overpressure, the γ phase’s volume fraction decreases due to the sizeable elastic strain energy. Furthermore, the volume fraction of the γ phase under differential stress is more significant than under static conditions due to the considerable shear plastic strain. The grains of the γ phase under differential stress at low confining pressure are lens-shaped with a strong preferred orientation normal to the maximum compression direction because of the shear plastic strain. Meanwhile, the grains of the γ phase at a high confining pressure are ultra-thin because of the considerable elastic strain energy.