Many industrial facilities, especially those in the chemical, oil & gas and petroleum industries, involve inherent risk in operations due to the processing of material which are hazardous in nature. It is therefore, necessary to precisely identify & analyze the hazards, its operability issues, associated risks and its consequences. Safety instrumented systems (SIS) are often used to reduce the risk associated with process or plant to the acceptable or tolerable level. The reliability of safety function(s) implemented through SIS is determined by the magnitude of risk reduction and is provided in terms of safety integrity level (SIL). The determination of SIL is the method of determining the risk reduction magnitude to safety instrumented function (SIF). There are several techniques used to establish SIL to SIF. These are qualitative, quantitative or mix of both, based on the application under consideration. Layer of protection analysis (LOPA) is one of the prevailing methods to determine the SIL. This paper aims to establish when, why and how to apply the LOPA to determine the safety integrity level (SIL) and finally reached to the conclusion that LOPA is one of the best method for determine the SIL.