Carbon fiber (~5mm long)-reinforced mortar is found to be an effective thermal sensor. In this paper, relations between temperature change and resistivity of carbon fiber- reinforced mortar (CFRM for short) with different carbon fiber contents (0.4%~1.2% by mass of cement) are studied. The results show that during the initial period, the resistivity decreases when the temperature increases (Negative Temperature Coefficient effect). After the temperature reaches a certain value, the resistivity increases when the temperature increases (Positive Temperature Coefficient effect). Besides, with the change of carbon fiber content, the transit temperature of NTC/ PTC effect also changes. Based on the experimental results, the CFRM shows a potential use as a thermal sensor. The mechanisms of temperature- sensitive properties and NTC/ PTC transition are also discussed.