This paper assesses the design guidelines of different classification codes for the liquefied natural gas carriers considering the effect of liquid sloshing. With regard the increasing importance of LNG carriers in today’s marine transportation, and also considering that the marine accidents involve significant loss, classification societies by regulating national and international standards aim to minimize the related risks. On the other hand, currently, industrial projects are being run in Iran for the design and construction of LNG carriers, and thus, the study of the liquid sloshing effect on the maneuvering ability of those ships is of prime importance. The present paper compares the approach of different marine classification codes in dealing with the sloshing effect in the design of LNG tanks. The outcome of this research is to introduce an integrated and complete procedure for the sloshing analysis.