Heat-resistant Ti-Al-Nb-Zr alloys, which don’t contain Sn, have been designed to obtain good oxidation resistance above 600 °C. In addition, to design Ti alloys with best balance of creep and fatigue properties, prior β grain size which affects fatigue properties and lamellar microstructure which affects creep properties were controlled by heat treatment. In the present study, the effect of microstructure on creep properties of one of the alloys, i.e., Ti-7.5Al-4Nb-4Zr alloy, with the bimodal (B), the lamellar structures in small prior β grains (LS), and the lamellar in large prior β grains (LL) were investigated at 600 °C. The creep deformation mechanism for each microstructure was a powerlaw creep. However, the creep life varied depending on the microstructures. The longest creep life was obtained in LS with prior β grain size of 90 µm and interlamellar spacing of approximately 10 µm, while the shortest creep life was obtained in LL with prior β grain size of 550 µm and fine interlamellar spacing of less than 2~3 µm. This suggests that creep life is more affected by interlamellar spacing than by prior β grain size.