Abstract Sputtered NiTi thin films are commonly deposited in an amorphous state and require a high-temperature annealing step to crystallize them. Their crystallization process is driven by the kinetic interplay of both nucleation and growth, and their resulting microstructure dictates the martensitic transformation behavior and actuation properties. In this paper, we present the full description of the individual contributions of nucleation and growth. Using in situ transmission electron microscopy heating methods with the Johnson–Mehl–Avrami–Kolmogorov, theory, we are able to determine these key parameters. During heating, we observed and analyzed the microstructural development and found a correlation between the average grain size and temperature. With cooling, we observed directly the conversion of the high temperature phase (austenite) to its low temperature phases (R-phase and martensite). We also examined the effect of composition on the martensitic transformation and found the crystallization behavior was markedly different for small variations. Such insights provide details that enable the continued development of thin film materials with optimal properties.