The low carbon bainitic steels gain increasing attention due to their high strength, high toughness, and good weldability. To improve the toughness and weldability of this kind of steel the carbon concentration is usually deduced to below 0.06% (mass fraction). As a result the hardenability of the steel is decreased and the ferrite usually becomes the first phase formed during the cooling process before the austenite transforms to the bainite. To decrease the nucleation activation barrier the grain boundary ferrite prefers to nucleate at the prior austenite grain boundaries, which are also potential nucleation sites for the bainite. The prior austenite grain boundaries are occupied by the ferrite, meanwhile ferrite/austenite interfaces are formed, which may influence the following nucleation of bainite. To understand the effect of grain boundary ferrite/prior austenite interface on the nucleation of bainite, a low carbon Fe--C--Mn--Si steel was investigated using optical microscope and electron back--scattering diffraction (EBSD). The grain boundary ferrite and bainite were formed during the two--step isothermal holding. By combining metallographic observation with orientation measurement, two kinds of interfaces were found between grain boundary ferrite and bainitic ferrite: one is non--clear interface, and another is clear interface. The analyses show that grain boundary ferrite has nearly the K--S orientation relationship with the prior austenite on the non--clear interface side, at which bainite nucleates and grow with an orientation similar to the grain boundary ferrite, while the grain boundary ferrite has a random orientation relationship with the prior austenite on the clear interface side, and large misorientation exists between bainite and grain boundary ferrite.