An investigation was undertaken to examine the fatigue characteristics of unidirectional dual fiber ceramic matrix composites made from Nicalon and SCS-6 fibers in UTRC-200 lithium aluminosilicate (LAS) glass matrix. Damage evolution was monitored as a function of fatigue cycles and stress levels via longitudinal stiffness loss. The damage consisted of matrix cracks, fiber-matrix disbonds, and broken fibers. Magnitude of damage was found to be strictly dependent on applied strain level. The consequence of damage development is manifested as stiffness loss for the composite. The stiffness degradation was monitored carefully, and a trend was observed. Based on the observed trend, a stiffness reduction model as a function of fatigue life was proposed that accounted for applied maximum strain and initial damage based on the first cycle maximum strain. The model was shown to be quite effective in predicting the damage evolution and thus fatigue life of dual fiber ceramic matrix composites (CMCs). The proposed fatigue life model is a generic one and can be applied to other CMCs exhibiting stiffness reduction as a result of multiple complex damage modes.