A new, simple and accurate method is proposed for the determination of the effective electron mass in thin insulator films from the current-voltage characteristics in fields of the ballistic electron emission regime. The effective mass of electrons was evaluated for oxide films of metal-oxide-semiconductor structures. The effective ballistic electron mass near the conduction band edge of silicon dioxide for different structures changes from 0.52 to 0.84m 0 and the experimental data show that the value of the effective mass of the ballistic electron can be treated as a constant while the gate voltage changes at least within a certain voltage range.