The effect of the nitrogen on the microstructure and properties of a cast Co-based superalloy K640S was investigated with the different level ranged from 24 ppm to 85 ppm. The results indicate that almost all nitrogen exist in the form of nitride precipitation in the vacuum induction melting. It is found that high N level would extend the solidification window during the solidification process, and then increase the quantity of the carbide eutectic phases and led to the distribution in the network form of the eutectics in the interdendritic regions. High nitrogen content (more than 40 ppm) can reduced the mechanical properties of the testing alloy. So, it is suggested that the nitrogen in the recycled Co-based superalloys should be controlled at a suitable level in order to avoid the degradation of their properties.