Twins in growing crystals are due to excessive thermal stresses induced by the temperature gradients developed during the growth process. Twinning is an important defect in advanced semiconductor crystals such as GaAS and InP. The objective of this study is to develop a computational model to predict the twin formation in the Gallium Arsenide (GaAs) crystals grown by the vertical gradient freeze method (VGF). A quantitative quasi-steady state thermal stress model is developed here for predicting the twinning formation in GaAs grown by VGF. The thermoelastic stresses in VGF grown crystal are calculated from a two-dimensional finite element analysis. Deformation twins form as a result of the high shear stresses acting on the twinning plane and in twinning direction. In the study, the resolved sheer stress (RSS) distributions in the twin systems for different growth directions have been calculated. This investigation is expected to further the understanding of twinning formation during crystal growth for different growth direction.