An investigation was undertaken to characterize the deformation response of unidirectional SCS-6/Ti 15-3 metal matrix composite (MMC) lamina subjected to in-phase thermomechanical fatigue (TMF) loading. Evaluations were done in an inert atmosphere to avoid oxidation effects. It was found that the dominant effect on deformation was due to strain ratchetting. Distributed fiber cracking was found to be the primary damage mode in in-phase TMF. The early part of strain ratchetting was identified to be due to the viscoplastic response of the composite as fewer fiber cracks were found to develop at this stage in the MMC specimens. Early fiber cracks were found to occur near molybdenum (Mo) ribbon sites. The fiber-cracking process was determined to be progressive in nature. On qualitative grounds, the concept of fiber ineffective length and the zone of stress intensification at the neighboring fiber at temperature, as in the case of in-phase TMF, was consistent with the observation of progressive fiber cracking in the MMCs.