The methods of optical, electron, and atomic force microscopy are used to study the morphology of bainite formed in high-strength structural steel 25G2S2N2MA (HY-TUF) under isothermal quenching. It is shown that the temperature of the transformation affects the substructure of the bainite, which is represented by ordered layers of plates with a width obeying a lognormal distribution law. It is shown that the bainite plates consist of nanosize subplates, the size of which is determined by the temperature of the bainitic transformation.