Single_phase iron pyrite (FeS22) crystalpowder was prepared by so l_gel hydrothermal method, which was characterized via x_ray diffraction, Fourier transform infrared and scanning electron microscope. A simplified mathematic model was established to analyze the crystal growth process. Results indicate that phase transformation and diffusion mechanism control the process, which fits the secon d_order kinetic equation with an exponent 3215 for time t. This is a decel erated process that can be divided into three steps: during the first period, ma rcasite and pyrite phases are produced, and then with inducting of sulfur into t he reaction, the process is dominated by the merging of marcasite into pyrite. U ltimately, with the fuse of small grains and the growing of larger ones, single_ phase iron pyrite crystals grow up through diffusion mechanism.