CF Thornel® T650 8-harness satin weave fabric composite with thermosetting polyimide NEXIMID® MHT-R resin designed for high service temperatures is produced at around 390°C and therefore high thermal stresses develop after cooling down to room temperature. Thermal transverse stresses in bundles/layers are tensile and lead to multiple intra-bundle /intra-laminar cracking. When the composite plate is subjected to large and repeated temperature variations, new cracks can appear due to thermally induced fatigue stress. Experimental results show that the highest temperature inthe cycle, where thermal stresses are low, has a significant detrimental effect on thermal fatigue resistance. Another observed phenomenon is thermal aging: at high temperature the mechanical properties are degrading with time. Aging and fatigue effects were separately analyzed for quasi-isotropic laminates with lay-up [(+45/-45)/(90/0)]2s.