In-phase thermomechanical fatigue (TMF) and elevated temperature isothermal fatigue (IF) experiments were conducted on a [0,±45,90]s Ti 15-3/SCS6 composite under load control. A correlation was obtained between the stabilized mechanical strain range and TMF life. TMF life was found to be significantly shorter than IF life when comparisons were made either on a stress-range or a gross mechanical strain-range basis. Damage modes were investigated using optical and scanning electron microscopy. The primary damage mode in IF specimens was transverse microcracking oriented perpendicular to the loading axis. In TMF specimens, although similar damage was observed very close to the fracture surface, the primary damage modes were inter-ply delamination and fiber-matrix debonding. Limited experiments were performed where damage was monitored using a replication technique on the specimen edges. These experiments confirmed delamination and debonding in TMF as well as thermally cycled specimens. Wavelength dispersive spectroscopic (WDS) analysis showed carbon-rich zones along many ply-to-ply interfaces, and these zones may have accelerated delamination cracking. Finite element analysis indicated that the maximum delamination stress occurred at the minimum temperature in the TMF experiments. The fatigue data of Ti 15-3/SCS6 composite, from this and other investigations, were plotted in the form of 0° fiber stress range versus logarithm of fatigue life. This phenomenological plot indicated a similar trend for various data sets, but the exact mechanisms remain to be determined.