Recently, modern long stroke marine diesel engines are adopted for energy saving. While this type of slow running diesel engine with small cylinder number has the advantage of shortenning of engine room length and simpler maintenance, vibration of the engine column should be considered. A longitudinal vibration has been observed on the engine column of 5-cylinder long stroke diesel engine which is installed in 80, 000 DWT oil tanker built at Mitsubishi Nagasaki ship yard. In this paper, the characteristics of engine vibration is studied as a first report. The longitudinal vibration of engine column can be calculated by FEM and mode synthesis. Calculated results show good agreement with the measured frequency. The parametric study is carried out by mode synthesis and simplified equation for frequency calculation is derived.