Abstract: The effect of added masses caused by inner liquid on low natural frequencies of hull＇s vibration was studied through experiment. The experimental object was a steel ship model. 306DF structural modal analysis system was used to do experimental research. Low natural frequencies of hull＇s vibration under different loadings were obtained. According to Singularity Distribution Method, the values of added masses under different water level in cabins were obtained. These added masses were taken into consideration when did the structure finite element calculation. Through the comparison between modal experimental values and finite element calculation results, the conclusion can be drawn that the added masses caused by inner liquid play an important effect on hull＇s vibration. The effect should not be ignored.