The mechanism of creep-fatigue interaction in a nickel base alloy (Nimonic 75 type) has been studied by transmission and scanning electron microscopy. Experiments at 873 K and a stress amplitude 392 MPa showed that, the data of creep, fatigue and "creep combined with fatigue" obeyed approximately the linear cumulative law. Micrographs of fracture surfaces by SEM indicated that, the rupture was all intergranular. However, observations by TEM showed that, the dislocation configurations within fractured specimens were quite different among creep, fatigue and "creep combined with fatigue". We believe that different dislocation configurations have different influence on the process of intergranular rupture, and the damages due to creep and fatigue are independent with each other. Consequently, linear interaction between creep and fatigue occurs macroscopically. The present paper shows that TEM has great potentialities for revealing the mechanism of creep-fatigue interaction.