The requirements on the design of SISs (Safety Instrumented Systems) based on SIL (Safety Integrity Level) have been developed continuously in the offshore industry. IEC 61508 and IEC 61511 illustrate various methodologies to determine the target SIL for specified safety functions, such as risk graph, layer of protection analysis, etc. These methods could arrive at different target SILs for the same safety function, mainly due to uncertainty in the models. In addition, uncertainties in the input parameters contribute to uncertainty in the target SIL In the offshore industry, engineers usually utilize two or more methods to assess target SILs for the same function and take the most conservative value as the target SIL In this paper, we investigate on the uncertainty in determining target SILs evaluated by the risk graph method, Minimum SIL Table from OLF 070 and LOPA. A procedure of SIL determination accounting for uncertainties is proposed for the risk graph method, Minimum SIL Table from OLF 070 and LOPA by using a Fuzzy Set approach only and the combination of Monte Carlo simulation and Fuzzy Set approach.