For the growth of large synthetic diamond crystals by temperature gradient method (TGM), the grit sizes of seed crystals have great effects on the growth rate and quality of large grown crystals. Because of the limited area of seed surfaces, the maximum diffusion flux of carbon source, which could be absorbed by the seed, is related to the seed size. And with increasing the seed sizes, the growth rates also increase markedly. However, the seed sizes should be lower than a certain value, which determines the crystal quality directly. For example, with NiMnCo alloy as the metal solvent, when the seed size increases from 0.5 to 1.8 ram, the growth rate increases greatly from about 1.1 to 3.2 rag/h; when the size is beyond 2.0 mm, more and more metal inclusions would be incorporated into the grown crystals, and the crystal quality is destroyed heavily. Finite element analysis (FEA) shows that, due to the special assembly of growth cell, the diffusion of carbon source in the metal solvent is very inhomogeneous, which could be substantiated directly by the appearances and shapes of large grown crystals and the remains of carbon source. And this inhomogeneous diffusion of carbon source would be very harmful to the growth of large diamond crystals, especially when large-size seed crystals are used.