The corrosion reinforcing steel in concrete structures is detrimental to its service life. In order to increase lifespan and maintain serviceability, a qualitative measurement technique that can measure the extent of corrosion of reinforcing steel needs to be developed. While considerable researches have been carried out focusing only on detecting corrosion and restraining the progress of corrosion, a qualitative measurement technique that can measure the amount of corrosion in rebar need to be developed. The purpose of this research is to develop a new technique to measure the corrosion level of rebar using accumulated thermal data. Accumulated thermal data were gathered using infrared camera and digitized to distinguish the difference between various corrosion levels of rebar. The test results showed that the higher level of corrosion displays the higher level of temperature. Temperature of reinforcing steel is not severely affected by ambient temperature, however, concrete surface temperature increased as ambient temperature increased.