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Upper bainite (B U ), lower bainite (B L ) and granular structure (GS) are three distinguishable microstructures formed through isothermal treatment at temperature range from 350 to 520 C in 15CrMnMoV steel. Although long time isothermal holding leads to the mixed microstructures, single microstructure can be obtained in the initial stage of isothermal transformation when the total transformation fraction is less than 20%. The Arrhenius equations were used to determine the overall activation energies of the transformations in the steel with accuracy of AE8 kJ/mole. To ensure the determined overall activation energy is the true value for single microstructure rather than for a mixture of two or more microstructures, the present determination of the kinetic data of isothermal transformations was carried out in terms of the isothermally transformed fractions of 1 and 5% at various temperatures. Experimental results show that the overall activation energy for single microstructure is significantly different from that for mixed microstructure, which was measured in terms of 50% transformation. Upon the determined kinetic data and the morphology of the microstructures, together with consideration of the reported atom diffusion activation energies, following conclusions are achieved. In steels, the B U transformation is controlled by carbon diffusion in the austenite; while, the formation of B L is only partially governed by carbon diffusion in austenite. As a different microstructure from other two, the GS transformation is controlled by carbon diffusion and has the common features of both equiaxed ferrite and massive ferrite transformations. These results, can be used to further understand the formation mechanism of B U , B L and GS in the 15CrMnMoV steel.