In this paper, boron diffusivities in HfSiON high-k gate dielectric films are systematically investigated for a wide range of the film composition. Boron diffusivities in HfSiON (1000°C) change from 3X10−17cm2/sec to 3X10−15cm2/sec according to the change in the chemical composition of the films. These diffusivity values are higher than that in SiON but lower than that in HfO2. We found that higher Hf/(Hf+Si) results in great increase of diffusivities. On the other hand, higher nitrogen concentration in the film leads to the reduction in diffusivity, in the case of the same Hf/(Hf+Si). In addition, crystallization of HfSiON with the same chemical composition gives rise to an anomalous increase in diffusivities, indicating that the micro-crystallized film structure of HfSiON critically affects the microscopic mechanism of boron diffusion in this material.