For the requirement of the International Convention for the Control and Management of Ships' Ballast Water and Sediments, the related marine microorganism growth tendency in the ballast water tanks for oil tanker has been investigated in order to find the way of the solution. In this study, the sample water from the ballast water of 100 thousand-ton oil tank will make the examination immediately on the ship. This will guarantee the data the authenticity, and will do for the following research reference. According to the examination result, ballast water is not to contain vibrio cholerae and the Escherichia coli, the intestines enterococci and the total colony number grows along with the storage time increases, but certainly not unlimited growth, after it is achieved the peak, returns immediately falls.