As the multimedia handheld devices using NAND flash memory as storage media is becoming more popular, the importance of the flash file system is increasing. Although there has been substantial research on file systems dedicated to NAND flash memory, little attention has been paid to legacy file systems running on top of NAND flash memory. In this paper, we propose an anti-fragmentation cluster allocation (AFCA) scheme for the Linux Ext2 file system. For the proposed scheme, we re-define the concept of file system fragmentation that takes into account the characteristics of NAND flash memory. The proposed AFCA scheme tries to minimize the performance degradation resulting from the fragmentation problem by discriminately allocating free clusters to files depending on their size relative to the flash memory block size. Evaluation results show that compared to the original cluster allocation scheme in Ext2, the proposed AFCA scheme significantly reduces the file system fragmentation according to the new definition and improves the file system performance, especially the write performance, by up to 43%.