Rational dither modulation (RDM) watermarking was presented to resist amplitude scaling attack. This property is achieved by quantizing the ratio of consecutive samples instead of samples themselves. In this paper, we improve the performance of basic RDM watermarking to resist more types of watermarking attacks. We improve the robustness of our modified RDM watermarking by the following three aspects: 1) The quantization step size is increased by modifying two coefficients instead of only one coefficient in the basic RDM method, 2) Several modification rules are defined to reduce embedding distortion, and 3) The coefficients with larger magnitudes in the lowest sub-band in DWT domain are selected to embed watermark. A variety of attacks are implemented to evaluate the performance of our method. Experimental results demonstrate that our method outperforms the basic RDM method and two state-of-the-art watermarking methods over a wide range of attacks and it also has good imperceptibility.