Physically Unclonable Functions (PUF) are hardware blocks implemented for challenge response generation used in many applications including cryptography. Use of hardware block instead of software application helps in improving security matrices PUF can be designed using different types but use of Configurable Ring Oscillator (CRO) block provides many advantages including reduction in area and improved performance matrices. Use of arbiter provides a simple design approach and less area compared to comparators used in the ring oscillator block. A configurable ring oscillator block with arbiter has application in key generation block of many crypto graphical units is proposed in this paper. Monte Carlo simulation used to study the effect of process variation in a design. In this paper Monte Carlo simulation is carried out on CRO block to obtain the results. © 2018 IEEE.