The phase separation process in a three-dimensional lattice gas model with a low volume ratio c=0.01 was studied. Using Monte Carlo simulation, the evolution behaviors of sc, fcc, bcc lattices, and 5 relative temperatures T/Tc=0.450.85 were simulated. It was found that the structure factor functions did not satisfy the scaling relationship, and the growth index was 1/6, less than the value of Lifshitz-Slyozov theory.