The purpose of this study is to develop clogging free casting of a rare earth (RE) alloyed steel. The experimental procedures included alloy design of RE-containing steel and production trials in the mill. We conducted a series of trials using various sulfur content in the steel before alloying of RE in the ladle and observed if clogging takes place during casting. We also sampled the steel products to determine the sulfur and oxygen contents. The deposits from the clogging were also sampled and analyzed using SEM/EDS techniques. Results showed that the clogging was closely related to the RE and S contents in the steel products. Clogging occurred in steel with RE content 0.017 - 0.036% and S content ≧ 20 ppm. The deposits from the clogging also contained a large amount of S, Ce, La, and Nd. The O content did not appear to be a factor effecting the clogging. We conclude that desulfurization to an S content ≦ 15 ppm before alloying of RE in the ladle should eliminate clogging in casting.