Surrounding neighbor blocks dependent fragile watermark scheme has its disadvantages, which couldn't successfully distinguish where the image has been tampered. In this paper, we propose a random block dependant fragile image watermark scheme. Two series, one is generated by this block and its prior block, the other is generated by this block itself, are embedded into the LSBs of this image block to keep itself secure and dependent on its prior block, which make VQ attack impossible. Decision strategy is implemented to detect whether the image blocks are authentic in tamper detection. Analysis and experimental results demonstrate that our algorithm can detect even one bit image alteration with graceful localization, and successfully resist vector quantization attack simultaneously without the need of any unique keys or image indexes.