The kinetics of crystallization of amorphous Fe40Ni40B20 alloy upon isochronal annealing was investigated applying power-compensating differential scanning calorimetry with heating rates of (5, 10, 20, 30, 40) K/min. The corresponding microstructural evolution was studied by means of X-ray diffraction, focused ion beam and scanning electron microscopy, and transmission electron microscopy. Crystallization of Fe40Ni40B20 alloy upon isochronal annealing takes place by formation of nano-scaled grains consisting of a face-centered cubic solid solution phase (Fe,Ni) and an orthorhombic compound phase (Fe,Ni)(3)B. Kinetic analysis was performed by application of a modular model of phase transformation kinetics, fitted to all experimental transformation-rate curves simultaneously. The crystallization reaction can be described by nucleation with a continuous nucleation rate incorporating a nucleation index a and by growth in three dimensions according to a linear growth law. The kinetics of transformation and the resulting microstructure observed upon isochronal annealing clearly differ from those upon isothermal annealing investigated in a previous study, reflecting different mechanisms operating upon isochronal and isothermal crystallization.