Large diamond crystals were successfully synthesized by a FeNi-C system using the temperature gradient method under high-pressure high-temperature conditions. The assembly of the growth cell was improved and the growth process of diamond was investigated. Effects of the symmetry of the carbon convection field around the growing diamond crystal were investigated systematically by adjusting the position of the seed crystal in the melted catalyst/solvent. The results indicate that the morphologies and metal inclusion distributions of the synthetic diamond crystals vary obviously in both symmetric and non-symmetric carbon convection fields with temperature. Moreover, the finite element method was applied to analyze the carbon convection mode of the melted catalyst/solvent around the diamond crystal. This work is helpful for understanding the growth mechanism of diamond. Large diamond crystals were successfully synthesized by a FeNi-C system using the temperature gradient method under high-pressure high-temperature conditions. The assembly of the growth cell was improved and the growth process of diamond was investigated. convection field around the growing diamond crystal were been systematically by adjusting the position of the seed crystal in the melted catalyst / solvent. The results indicate that the morphologies and metal inclusion distributions of the synthetic diamond crystals vary obviously in both symmetric and non-symmetric carbon convection fields with temperature. Moreover, the finite element method was applied to analyze the carbon convection mode of the melted catalyst / solvent around the diamond crystal. This work is helpful for understanding the growth mechanism of diamond.