Physical Unclonable Function (PUF) is recently used in modern electronic systems for device authentication and secret key generation. SRAM PUF is a popular memory-based PUF because it is a standard component for most electronic devices. Previously only strongly stable RAM bits are selected as PUF bits in order to achieve high reliability, hence requiring relatively large SRAM. Furthermore, SRAM PUF might suffer from PUF clone attack for using stable bits. Attackers may use Photon Emission Analysis (PEA) to observe the behavior of the stable bits and conducted Focused Ion Beam circuit edit (FIBCE) to produce an identical physical response. In this paper, we propose a new method that employs unstable bits as PUF bits in addition to the stable bits to increase the SRAM bit usage rate. We also show that this method can efficiently resist the PUF clone attack. Experimental results show that even though our proposed method uses unstable bits as PUF bits, it can still achieve high reliability. It can reach an average 2% intra-HD without Error Correction Code (ECC) under various conditions and an average inter-HD of 49.33%.