The effect of small additions of boron and carbon on the environment embrittlement at different temperature of Ni-19Si-3Nb based alloy was investigated by atmosphere-controlled tensile test in various conditions. The results revealed that the Ni-19Si-3Nb base alloy exhibits ductile mechanical behavior (UTS∼1250 MPa, ε∼14%) at room temperature in vacuum (2×10-4torr) as well as in pure oxygen atmosphere. This indicates that environmental embrittlement due to water vapor is the major factor to the deterioration of the ductility of Ni-19Si-3Nb base alloy. In parallel, 300 ppm boron addition significantly improves the ultimate tensile strength and the ductility (UTS≥1200 MPa, ε≥12%) in the Ni-19Si-3Nb base alloy at room temperature in various atmosphere of pure oxygen, air (contains 14000 ppm water vapor), and pure water vapor (contains 850 ppm water vapor). In addition, the yield strength of the Ni-19Si-3Nb-0.15B-0.1C alloy increases with temperature, and the maximum yield strength occurs at 873 K. However, the ductility of the Ni-19Si-3Nb-0.15B-0.1C alloy drops significantly when the temperature increases to 973 K.