As a new topic in security area, Physical Unclonable Functions (PUFs) have many advantages against traditional techniques. They can be employed to generate unique signature of Integrated Circuit(IC) for identification and authentication, which is so called DNA of ICs. In this paper we introduce some basic knowledge of PUFs, as well as the structure and implementation details of Ring-oscillator PUFs. Then we propose an improvement of RO PUF, which is called Multiple Bits Output Ring-Oscillator PUF. The Multiple Bits Output RO PUF helps to generate more challenge-response pairs with very little extra area cost. Moreover, some methods for enhancing the reliability and uniqueness are used. The experiment results are showed at last.