Thermoelectric behavior and microstructure of carbon nanotubes/carbon fiber(CNTs/CF)- cement based composite have been measured in this study. An self-made experimental setup was applied to test the thermoelectric power (TEP) of the composites. The results show that the higher the CNTs content, the less positive the absolute thermoelectric power is. When CNTs addition incresed to 1.0% by weight of cement, the absolute thermoelectric power changed sign from positive to negative. Scanning electron microscopy (SEM) was used to characterize the morphology of CNTs, CF and the structure of Portland cement-CNTs-CF systems. SEM analysis of the results show that good interfacial adhesion between CNTs and cement matrix is seen with CNTs tightly wrapped by Calcium-Silicate-Hydrate (C-S-H). With the incorporation of CNTs/CF in cement based composite, the cement-CNTs-CF system exhibits a porous microstructure.