Submicrometer-sized Ga particles were dispersed in polymethylmethacrylate (PMMA) matrix by an ultrasonic vibration and sedimentation method. The solid phase transition from γ-Ga to δ-Ga and its Ga particle size dependence were studied by means of differential scanning calorimeter measurements. It was shown that a solid–solid phase transition corresponding to the -Ga one happened in Ga particles upon cooling. Moreover, the ratio of the particles undergoing the solid phase transition to all particles increases with decrease of the particle size.