Low cost silicon carbon nanometer powder was synthesized by carbothermal reduction method with nanometer SiO2 and carbon as raw material. Its synthesis thermodynamics were discussed. The influence of La on TG-DSC curve was also analyzed. It indicated that the synthesis process of SiC powder had two steps. In the first step two medial productions of SiOg and COg formed, and in the second step, β-SiC was finally synthesized. After 0.3% La added, at the first step, the initiatory forming temperature of producing SiO(g) and CO(g) declined from 1351.4 to 1250.9℃, and the thermal activation energy decreased from 223.6 to 34.7 J·g-1; at the second step the initiatory forming temperature of synthesizing β-SiC powder declined from 1526.5 to 1357.8℃, and the thermal activation energy decreased from 693.7 to 295.7 J·g-1. Without La added, the best synthesis technology for β-SiC powder was 1550℃ for 120min, average powder diameter was bigger about 150nm.With La added, the best synthesis technology was 1500℃ for 120min, average powder diameter was about 100nm.