Damage sensing of the fiber-reinforced structural composites have attracted a lot of attention. In this work, a small amount of carbon nanotubes (CNT)-Al2O3 hybrids are introduced into the woven glass fabric reinforced epoxy composites and serve as in situ sensor to monitor the damage initiation and propagation under mechanical loading. The hybrids with CNTs grown on the Al2O3 micro-spheres are synthesized by chemical vapor deposition. The addition of 0.5 wt.% CNT-Al2O3 increased ac electrical conductivity of the glass fabric/epoxy composites at 103 Hz 4-5 orders of magnitude in both in-plane and through-thickness directions. The electrical resistance of the composites was in situ measured under quasi-static tensile testing. With the increase of strain, the resistance response could be classified into three distinguished stages, corresponding to various damage modes (microcracks, fiber/matrix interfacial debonding, transverse cracks, delamination, fiber breakage, etc.).