By diffusion of water and other factors which cause cathodic disbondment in the polymeric coat and reaching metal/polymer interface, hydroxyl is produced and makes the interface alkaline. The alkalinized interface causes disbondment of polymeric coat from metallic substrate. The scope of this project is to evaluate the cathodic disbondment of polyurethane coat which is used to protect oil and gas pipe lines. The quantity of cathodic disbondment of a polymeric coat depends on two parameters. These two parameters are adhesiveness of polymeric coat to metal substrate and resistance of polymeric coat to diffusion of water and other factors which cause cathodic disbondment. For having a valid study on cathodic disbondment phenomenon, these specifications should be evaluated. In this project, according to Iranian gas standards, these parameters are studied. The resistance of the polymeric coating to diffusion of water and other factors is studied by two quantitative tests including water absorption test and chemical resisitance test performed in acidic solution, alkaline solution, pure water and saline solution. o the amount of absorbed water and reagents in saturated condition is measured and compared to the standards. Also, adhesiveness is studied by two qualitative test including resistance to removal test and hot water soak test and one quantitative test including pull-off strength of coatings using portable adhesion testers. It is shown that both specifications of the polyurethane is in the standard range. After investigating these two parameters, cathodic disbondment is measured. For evaluating cathodic disbondment, a simple electrochemical cell is set up and specimen is connected to negative terminal of a rectifier and the positive terminal is connected to a platinum electrode, so the specimen is under cathodic protection for a assessed period of time. The measured quantity of cathodic disbondment is more than what assessed in the standard. Noting that the resistance of the polyurethane to diffusion of water and other reagents and also adhesiveness of polymeric coat to the metal substrate are in the standards range, it can be concluded that surface unevenness of polymeric coat, created by using a brush for painting, causes residual stress in polymeric coat. The created residual stress aids the disbonding of polymeric coat from metal substrate and increases the amount of cathodic disbondment. One of affective parameters on cathodic disbondment is the thickness of polymeric coat. For evaluating the parameter of thickness, cathodic disbondment and coat resistance of three different polyurethane thickness are measured. Three double electrochemical cell is set up for this test to measure the polymeric coat resistance and the quantity of cathodic disbondment coincidely. Specimens as the chathodic disbondment test are under cathodic protection for the same period of time. It is shown that increasing the coat thickness, will increases the polarizatio resistance and pores resistance of olymeric coat and the quantity of cathodic disbondment decreases. Also by electrochemical test is shown that pores of a polymeric coat have an important role in efficiency of coat. Increasing the amount of pores of a polymeric coat, will increase the quantity of cathodic disbondment. So it is concluded that increasing the thickness of coat, will increase the resistance of polymeric coat and reduces the cathodic disbondment of a non porous polymeric coat. key words Cathodic Disbondment, Cathodic Protection, Polyurethane, Pipe line