The sliding gate plate that controls steel flow through the tundish nozzle sometimes cracks leading to air aspiration and safety concerns. To evaluate possible mechanisms for crack formation, this research applies a 3-D finite-element model of the thermal and mechanical stress in a sliding-gate plate during preheating and casting induced by thermal expansion and/or mechanical movement. The thermal model is first validated with previous temperature histories measured during preheating and casting in a ladle plate. The model of a tundish sliding gate nozzle is then used to investigate thermal-mechanical behavior and cracking due to the temperature variations during preheating and casting. The model predictions of the maximum stress location and orientation match well with the crack location observed in used plates from POSCO. Different mechanisms for the formation of two different types of common through-thickness cracks are identified and explained. The first involves exterior tensile stress during heating stages, and the second is due to excessive compression from non-optimal placement of guide points on the steel cassette.