The interfacial energy between the gamma and gamma ' phase in Ni-Co base superalloy TMW-4M3 was investigated using both a kinetic analysis of experimental data and a thermodynamic analysis. The coarsening rate of gamma' precipitates during the aging treatment process was analyzed using extended Lifshitz-Slyozov-Wagner theory, including the contribution of a full diffusion matrix with a thermodynamic calculation. The interfacial energies calculated from the coarsening kinetics using only the diagonal components were half as large as those using the off-diagonal contributions as well. The interfacial energy with full diffusion matrix contributions is about 75 mJ/m(2) between 1373 and 1413 K. In addition, the interfacial energy was also analyzed on the basis of the extended Becker's model (EBM), and the results, which ranged from 73.9 to 76.3 mJ/m(2 ), showed good agreements with those from the coarsening kinetics. A comparison of the interfacial energy between TMW-4M3 and several Ni base superalloys indicated an identical tendency of higher interfacial energy with a larger Ti/Al ratio. The off-diagonal terms of a diffusion matrix are very important in analyzing the coarsening kinetics of gamma' and gamma/gamma' the interfacial energy estimation using the EBM is a suitable method for TMW-4M3.