Abstract The growth rate of iron whiskers grown from ferrous chloride by Brenner's method was measured and the growth morphology was observed. The axial growth tended to be time-saturated independent of the growth temperatures. The radial growth started after an induction period. At a low growth temperature the axial growth rate corresponds to theoretical predictions. At higher growth temperatures the experimental rate is lower than the theoretical one. This discrepancy is successfully interpreted in terms of the halide vapor pressures of adsorption which are responsible for the presence of an impurity of the growth site at the whisker's tip. It is indicated that the tip growing whiskers of copper reported by Régis et al. is also due to the same mechanism. The radial growth rates are about one hundredth as large as the axial growth rates and presumably rate-determined by the halide-vapor transport process of CVD.