Nano-sized amorphous nickel sulfide has been prepared in a mixed aqueous solution of Na2S2O3 and Ni2SO4 compounds using r-irradiation at room temperature. The obtained precipitate after r-irradiation was washed and dried at 60℃. It was then crystallized by calcination in a now of ar n at 160 -- 500℃, respectively. All these samples were characterized by X-ray diffraction and extended X-ray absorption fine structure (EXAFS) spectroscope. No peak is detected in the X-ray diffraction patterns of 60℃ dried and 160℃ calcined samples, it reveals that particles in these two samples are in amorphous states with high level of disorder. Several weak peaks are found in the pattern of 300℃ calcined sample and they become stronger in that of 400℃ and 500℃ calcined samples. All these peaks are due to NiS compound. EXAFS results demonstrate that the nickel sulfide particles in 60℃ dried sample are present in a tetrahetral structure with an average Ni -- S coordination distance of 0. 221nm and a Ni - S coordination number of 4.4, which are changed to octahedral structures with Ni - S coordination distances of about 0. 238nm and Ni - S coordination numbers of near 6.0 after calcined at above 300℃. Both the tetrahedral species and the octahedral species exist in 160℃ calcined sample.