Effect of increasing mould wall thickness on quality of low alloy square steel ingot of 40cmd8 grade (AISI P20) and of 4 t weight were studied by comparing three cases with the help of FEM simulation. Solidification simulation software THERCAST was used which could predict fluid flow, temperature, solidification, piping and centreline porosity for the cast ingot. With increased mould wall thickness fluid flow was not much affected. Heat transfer was retarded with increased mould wall thickness which delayed the solidification. Solidification profiles were with thin mushy zone and wide shaped which has lesser chances of dendrites bridging and helps in avoiding central porosity. Piping and porosity were predicted based on Niyama criteria. It was seen that porosity in the ingot centre were decreased with increased mould wall thickness. Optimal mould wall thickness could be obtained for square ingot of low alloy steel grade.