Nanocrystallization by heat treatment of initially amorphous alloys occurs for high nucleation frequency I and low crystal growth rate u values. Calorimetrie data extracted, for instance, from differential scanning calorimetry are normally presented by the dependence of the crystallized fraction x, on: i) x=x(T,t) when obtained under isothermal annealing at temperature T as a function of time t; ii) x=x(τ,β) when obtained under continuous heating at a scan rate β as a function of temperature. Theoretical analysis of the crystallization kinetics is presented which accounts for nuclei either pre-quenched or created by homogeneous nucleation whose initial steps of growth are controlled by the interface formation between the nanocrystals and the matrix and subsequent growth is limited by diffusion.