In this study, reinforcement steel corrosion is monitored with OFDR（Optical Frequency Domain Reflectometer） distributed optical fiber. Three cylindrical mortar specimens containing 12 mm, 16 mm and 20 mm diameter steel bars were fabricated in the lab, and optical fiber was wrapped around the mortar cylinder in a spiral mode with a spacing of 100 mm. All three specimens were subjected to accelerated corrosion test. During the accelerated corrosion test, strain data on the mortar surface induced by steel corrosion were collected with OFDR optical fiber. Results show that corrosion of steel bars causes surface cracking of mortar cylinder and results in appearance of strain peaks along the OFDR fiber. The magnitude of strain peak increases with an increase of corrosion-induced mass loss of steel bar. At the same corrosion level, bigger steel bar causes a higher cracking rate and consequently a bigger crack width. Therefore, OFDR distributed optical fiber can be used to effectively monitor corrosion of steel reinforcement.