A method for defect management of a rewritable optical disc is provided. The rewritable optical disc has a data area comprises recording units for storing a target data and a replacement area comprising first replacement units for replacing defective recording units of the data area. First, a fragmented status of the target data is calculated, wherein the fragmented status reflects a dispersion level of the first replacement units in view of a distribution of the defective recording units. The fragmented status is then compared with a threshold value to determine whether to perform a defragment operation about the target data. Finally, the defragment operation about the target data is performed by rearranging the first replacement units in accordance with the sequence of the defective recording units.