This work aims at analyzing process‐induced distortions (PIDs) for unidirectional thermoplastic composites in the thermoforming process and evaluating the effects of fiber reorientations and stamping‐induced stresses. Hemispheres with symmetric and asymmetric lay‐ups are manufactured by the thermostamping process. A simulation method of PIDs is developed and a constitutive model is presented to reflect fiber reorientations in thermostamping. Additionally, stamping‐induced stresses are embedded in the simulation. Good correlation is obtained between the prediction and experimental results. Regarding hemispheres with the symmetric lay‐up [0/90/0/90]S, stamping‐induced stresses play a dominant role in PIDs and determine the deformation trend; fiber reorientations contribute approximately 76% to thermal distortions. Stamping‐induced stresses could be neglected in the PID prediction for the investigated three different asymmetrical lay‐ups, and contributions made by fiber reorientations range from 18% to 28%. Both fiber reorientations and stamping‐induced stresses are significant factors for PIDs for unidirectional thermoplastic composites in the thermoforming process. POLYM. COMPOS., 40:1786–1800, 2019. © 2018 Society of Plastics Engineers