An examination was carried out on the physical basis for the so‐called ’’extremum condition’’ which determines the dimension of transformed phases during the steady‐state cellular phase‐separation reaction. The concept of the ’’balance of forces’’ which employs the balancing of various forces at the moving phase‐separation boundary as the condition for the steady‐state cellular phase‐separation reaction is proposed. The magnitude and the direction of these forces are given by the partial derivatives of the total free‐energy reduction due to the phase change at an undercooled condition in terms of the relevant reaction parameters such as the size of the microstructure and the reaction velocity. A simple model of lamellar formation by the eutectic or eutectoid reaction is used to demonstrate the meaning of the extremum condition and also to show the usefulness of the balance‐of‐forces concept.