Once articular cartilage is damaged it cannot be readily repaired. However, to what extent marathon running causes cartilage damage to the knees is unclear. We quantitatively assessed the morphologic and T2* value changes in the knee cartilage of marathon runners using an automatic cartilage segmentation method. The cartilage volume, thickness, and T2* values of 21 sub-regions were quantitatively assessed. The results showed that the T2* value of knee cartilage increased right after running and recovered two months later, suggesting that the knee joint cartilage showed a degree of reversible change after marathon running.