Digital watermarking is a basic method for copyright protection. This paper presents a blind watermarking approach in DCT domain with Spread Spectrum technique. The method divides the image into non-overlapping blocks of size 8x8. Each block is transformed into frequency domain using DCT. Middle band frequency coefficients of each block are utilized to embed and extract watermark. Two number sequences of size 1x22 are carefully (first set in increasing order and the other one in decreasing order) predetermined by the algorithm to improve the robustness of the method against some attacks. The strength of the watermark is also determined dynamically according to the energy characteristic of the current block. Experimental results show that the proposed method can extract watermark even when an image was distorted by some attacks such as JPEG compression, cropping and noise addition. The method also has higher Normalized Cross-Correlation (NC) values compared to other works in the literature.