In recent years, some small and midsize gas fields are being developed all over the world, and FLNG systems which include LNG FPSO and shuttle LNG tank are going to be used. In rough sea, when produced LNG is offloaded from the FPSO to the shuttle tanker, two ships will be moored. In this study, we are focusing on characteristics of FLNG systems in waves and wind. Experimental studies were performedto observe motion of two floating bodies in tandem condition in regular waves. In the tank test, we observed drastic yawing and drift motions of a shuttle LNG tanker. These phenomena might be a threatto the safety of a FLNG system. In this paper, we will reveal a cause of these phenomena and evaluate a FLNG system by using a simple method.