In recent years, there has been tremendous interest in developing a highly integrated DNA analysis system using microfabrication techniques. With the success of incorporating sample injection, reaction, separation and detection onto a monolithic silicon device, addition of otherwise time-consuming components in macroworld such as sample preparation is gaining more and more attention. In this paper, we designed and fabricated a miniaturized device, capable of separating size-fractioned DNA sample and extracting the band of interest. In order to obtain pure target band, a novel technique utilizing shaping electric field is demonstrated. Both theoretical analysis and experimental data shows significant agreement in designing appropriate electrode structures to achieve the desired electric field distribution. This technique has a very simple fabrication procedure and can be readily added with other existing components to realize a highly integrated "lab-on-a-chip’ system for DNA analysis.