By contrast of microstructure and boron distribution in the same area of samples, the behavior of boron during ferrite reaction in Nb-B microalloyed steel was studied. Boron atoms have segregated to original austenite boundaries betfore ferrite reaction starts during isothermal treatment. The concentration of boron in small Pieces of ferrite foming along austenite boundaries may be remarkably higher than that in austenite matrix, but it will reduce gradually when the ferrite continuously grows. There is no obvious boron segregation on interface of γ/α and α/α.These facts indicate that ferrite growth is not controlled by diffusion of boron in austenite.