Drawing ideas from least significant bit, a quantum watermarking strategy was proposed. In this strategy, one embedder embeded a watermark image into some pixels of quantum carrier image by changing one bit of gray-scale value of carrier image, these pixels were determined by a private key. Only the copyright owner who held the private key could extract the watermark information without knowledge of the original carrier image. Quantum circuits of embedding and extracting algorithms were respectively designed. Moreover, the computer simulation of the proposed watermarking algorithm was given. By comparing the peak signal-to-noise ratio between the embedded carrier image and original carrier image, it indicates that the proposed watermarking strategy achieves invisibility. The experimental results also show that the proposed watermarking strategy is flexible.