Isothermal sintering behavior of pure molybdenum (Mo) and mechanically alloyed Mo–TZM (Mo–0.6Ti–0.2Zr–0.02C) has been investigated in the temperature range 1000–1800 °C. A linear relationship has been found to exist between logarithms of increment in density and time. Although the volume diffusion has been found to be the dominant sintering mechanism, a significant contribution from grain boundary diffusion is also identified. Both the diffusion coefficients ( D v ) obtained from shrinkage data and the grain boundary mobility ( M b ) during grain growth are found to be lower for Mo–TZM due to the presence of carbides in the microstructure. The grain boundary migration is restricted due to the presence of carbides and porosities in the microstructure.