A rapidly quenched nanocrystalline Hf11Ni89 alloy was produced by melt-spinning. The x-ray phase analysis shows that the as-quenched ribbon consists mainly of nanocrystalline fcc HfNi5 although a small amount of Ni(Hf) solid solution is also detected. The crystallite size distribution and the dislocation structure of the dominant HfNi5 phase were determined by a recently developed method of diffraction profile analysis. In this procedure, by assuming spherical shape and log-normal size distribution of crystallites, the Fourier coefficients of the measured physical profiles are fitted by the Fourier coefficients of well established ab initio functions of size and strain peak profiles. The anisotropic broadening of peak profiles is accounted for by the dislocation model of the mean square strain in terms of average dislocation contrast factors. It was found that the median and the variance of the crystallite size distribution are 3.3 nm and 0.82, respectively. The dislocation density is 3.7 x 10(16) m-2 and the character of dislocations is nearly pure screw. The results obtained from x-rays were in good agreement with transmission electron microscopy observations.