Based on sequentially coupled CFD and FEM models, aluminum alloy rolling ingot thermal stress simulations have been conducted in order to understand start-up phase cold cracking phenomena and optimize tooling designs for 520×2120 mm rolling ingot casting on Wagstaff® Epsilon™ Ingot Tooling. In the CFD model, ingot surface temperature dependant and water flow rate dependant water boiling curves are applied. Thermal boundary conditions for the complex water intrusion phenomena under the ingot butt have been attempted. Temperature dependant elastic-plastic materials constitutive relationship has been employed in the transient thermal stress FEM model. Results of thermal stress development at ingot surface and inside the ingot are presented; Connection of cold cracking (ingot butt quarter and center cracks) with near surface stress development at the ingot butt is shown and the effect of water intrusion under the ingot butt on the butt stress development is also discussed. The predicted temperatures are validated against temperatures measured from cast-in thermocouples at strategic locations in field ingots in order to obtain realistic thermal boundary conditions. The predicted butt curl is also verified through field observation and measurement.