Recent incidents such as the Stuxnet worm have shown that cyber attacks can cause serious physical damage of critical infrastructure.The paper presents a cyber physical attack dual-loop analysis model for formal attack modeling of the attack impact metrics for these types of cyber attacks.Then,various attack modes and the attack impact are evaluated on a chemical plant simulation system.The results show that traditional control methods which are robust to physical disturbances are vulnerable to current intentional cyber-physical attacks.The results also show the characteristic of various types of cyber-physical attack policies and parameters.