The recent growth of networked multimedia systems has caused the need for intellectual property rights, like images, music and movies. One approach to solve this problem is to add an invisible image to an image that can be used to provide solid proofs of ownership. These invisible images or signatures are known as digital watermarks. In this paper, we will describe a novel technique for watermarking. This technique is implemented in DWT domain. Different from usual watermarking methods, we use two sets of secret key systems, one is called private key and the other, public key. Experimental results show that our method is valid in embedding a watermark in an image and the watermark is robust enough to various attacks.