The method to measure boron profile by using the nuclear reaction10B(n,&agr;)7Li is applied to estimate the diffusivity of boron in the glow‐discharged hydrogenated amorphous–silicon (a–Si:H). It is found that the diffusivity of boron ina–Si:H is much larger than that in crystalline silicon (c–Si). For instance, it is about 2×1016cm2/s at 330 °C and larger than inc–Si by twelve orders of magnitude. The activation energy of the diffusivity is also estimated to be 1.5 eV. These values are nearly equal to those of hydrogen ina–Si:H. The minimum duration, in which the characteristics ofp–i–ntypea–Si:H solar cells may start to change due to this boron diffusion, is roughly estimated using both boron diffusivity and its activation energy.