Rod diameter of Type II manganese-sulfide inclusion precipitating in residual melt among steel dendrites is observed. The data about relationship between diameter, distribution/size of the inclusions and steel dendrite arm spacings were considered. The sample steels solidified under several steady state uni-directional solidification conditions. The consideration was done from a view point of cooling rate dependency of MnS-dendrite arm spacings, taking cooling behavior as well as shrinkage of the residual melt pool into account. The result in experiments that rod size of MnS-inclusions is correlated to Solidification-Unit-Cell volume is explained from cooling behavior of the residual melt pool.