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Crystallization kinetics of Fe55-x Cr18Mo7B16C4Nb x (x = 0, 3) bulk amorphous alloys were analyzed using X-ray diffraction and differential scanning calorimetric (DSC) tests. In practice, crystallization and growth mechanism were evaluated using DSC tests at four different heating rates (10, 20, 30, and 40 K/min) and kinetic models. Two-step crystallization behavior was observed when Fe55Cr18Mo7B16C4 and Fe52Cr18Mo7B16C4Nb3 bulk amorphous alloys were annealed, where Fe36Cr12Mo10 phase was crystallized in the first step of crystallization. Results show that Fe36Cr12Mo10 and Fe3C phases were crystallized in the structures of the alloys after further annealing process. Activation energy for the crystallization of Fe36Cr12Mo10 phase was measured to be 543 kJ/mol in Fe52Cr18Mo7B16C4Nb3 alloy and 375 kJ/mol for Fe55Cr18Mo7B16C4 alloy according to Kissinger-Starink model. Moreover, a two-dimensional diffusion controlled growth mechanism with decreasing nucleation rate was found in Fe52Cr18Mo7B16C4Nb3 alloy whereas a three-dimensional diffusion controlled growth mechanism with decreasing nucleation rate was found in crystallization of Fe36Cr12Mo10 phase during annealing of Fe55Cr18Mo7B16C4 alloy. TEM (transmission electron microscopy) observations reveal that crystalline Fe36Cr12Mo10 phase nucleated in the structures of the alloys in an average size of 10 nm with completely mottled morphology. [Copyright &y& Elsevier]