The concept of SIL is applied in the most of all standards relating to functional system safety. However there are problems for the people to apply SIL to their plants. as these standards don`t include sufficient informations. In this regards, this paper will suggest the direction of SIL application and concept based on IEC 61508 and IEC 61511. A Safety Integrity Level(SIL) is the discrete level(one out of possible fours), corresponding to a range of the probability of an E/E/PE (Electric/Electrical/Programmable Electrical) safety-related system satisfactorily performing the specific safety functions under all the stated conditions within a stated period of time. SIL can be divided into the target SIL (or required SIL) and the result SIL. The target SIL is determined by the risk analysis at the analysis phase of safety lifecycle and the result SIL is calculated during SIL verification at the realization phase of safety lifecycle. The target SIL is determined by the risk analysis like LOPA(Layer Of Protection Analysis), Risk Graph, Risk Matrix and the result SIL is calculated by HFT(Hardware Fault Tolerance), SFF(Safe Failure Fraction) and PFDavg(average Probability of dangerous Failure on Demand). SIL is applied to various areas such as process safety, machinery (road vehicles, railway application, rotating equipment, etc), nuclear sector which functional safety is applied. The functional safety is the part of the overall safety relating to the EUC and the EUC control system that depends on the correct functioning of the E/E/PE safety-related systems and other risk reduction measures. SIL is applied only to the functional safety of SIS (Safety Instrumented System) in safety, EUC is the abbreviation of Equipment Under Control and is the equipment, machinery, apparatus or plant used for manufacturing, process, transportation, medical or other activities.