With the rapid development of the Internet, multimedia communication and authorization become more and more important. In order to avoid geometric distortions in watermark embedding, this paper proposes a geometrical attack resistant image watermarking algorithm based on histogram modification. The watermark information is embedded into the original image by modifying the number of the gray samples of the image histogram. One bit of the watermark is embedded by changing the number of samples of the three adjacent grey levels in the histograms and the same watermark repeatedly embedded into each of the four histograms. Watermark detection is an inverse process of watermark embedding. Watermark information is detected based on the relationship of the number of samples of the three successive gray levels. The experimental results show that our algorithm is robust to both geometrical attacks and those conventional signal processing attacks and achieves high detection rate.