Configurable ring oscillator (CRO) physical unclonable functions (PUF) are used to improve the uniqueness and reliability of the conventional RO PUF. Previous researches have shown that multiplier, XOR gate and trsitate inverter based CRO PUFs can improve the uniqueness and reliability. However the efficiency is remain in the same level when compared with the conventional RO PUF implementation. In this paper, a programmable RO PUF (PRO PUF) is proposed, which can be programmed to change the structure of a typical RO PUF. The proposed PUF structure is implemented based on the switch matrix and can be programmed to be a chain like RO PUF or a random loop RO PUF. The new PUF structure can be applied to a FPGA system or a ASIC implementation. A FPGA implementation is given based on Xilinx Spartan 6. Xilinx design language (XDL) is used to access the programming interconnect point in the FPGA. Experimental results demonstrate that the PRO PUF has a good uniqueness, reliability and a very high hardware efficiency.