A driven roll measuring system has been developed to diagnose strand deformation and bulging during continuous casting and their effect on slab qualities has been discussed. In-cast roll gap profile in the casting direction could be predicted by driven roll displacement monitoring and was discussed regarding the frequency of transversal corner crack. Unsteady bulging, which is generally accompanied by steel level variation within mold and casting speed variation, has a tendency to occur for the steels with 0.09-0.12% C and to increase along downstream side. Frequency analysis which was performed on unsteady bulging showed that high frequency of 0.2-0.25 ㎐ as well as roll rotation and roll pitch frequencies occurred. These particular frequencies were discussed regarding the relation of unevenness of solidified shell. A mathematical model has been developed to simulate the unsteady bulging phenomena based on FEM analysis on the case of periodical casting speed change. Finally, the effect of unsteady bulging on slab qualities also has been discussed.