This paper describes a new watermarking attach algorithm, a screen mark attack. The screen mark attack is a modified overmarking attach, which attacks a watermarking system by Inserting another watermark into a marked Image. Overmarking attack has a problem that it cannot be applied to a public watermarking software that prohibits the watermark embedder from embedding another watermark Into an already marked image. However, the proposed attack algorithm can be applied to such a pub1ic watermarking scheme. Test results show that the proposed watermarking attack algorithm is successful for commercial watermarking softwares and attacked images show better quality than Images attached by 07her attack tools.