Encryption, digital signature and digital watermark are three popular methods to prevent a wide variety of multimedia products from illegal copy or subterfuge attacks. In particular, digital watermarking recently draws a lot of attention since it hides desirable information in transmitted audio, image and video data files without affecting much the data quality. The thesis proposes two watermarking methods for audio and image data. And both watermarking methods embed watermark signals in the spatial/time domain using a so-call Pixel Weight Table. The experiment results shows that the two methods have good performance. In addition, I propose a guantitive measuring method, called APSNR, to characterize the audio perceptual transparency.