Since the meniscus stability has a great influence on the final semi-product quality, it is essential to study how the meniscus behaves in aMedium-thin slab medium-thin slabThin slab mold with straight walls; because of that, the present research develops this important topic by mathematical modeling considering the governing equations, the k-e turbulence model, and the VOF model. All equations were solved under isothermal and transient conditions through commercial CFD software. The results show high meniscus fluctuationsMeniscus fluctuation with no symmetry between its right and left sides. This meniscus flow behavior is a consequence of the reduced space of this mold type and the high casting velocity, but mainly of a deficient SEN design. Since this mold type does not use the funnel mold shape, the jets transfer their unstable behavior toward the upper roll flows, inducing the detected high meniscus instabilities. The quantification identifies the left meniscus side as the more unstable.