A proposal of the nip zone angle determination in high-pressure grinding rolls (HPGR) crushing process is presented in the paper. Knowledge of the nip zone angle value makes it possible to determine the real maximum pressing force in the high-pressure comminution. Two methods of determination of this maximum pressure were proposed and verified. The first method is based on the roll’s geometry and the change of the HPGR chamber’s volume together with the roll rotation, while the second one considers the outlet gap width. Results of the nip zone angle calculations according to both methods are similar and show that the nip zone angle is around 3o. The verification was carried out in a piston-die press and experimental results show that the product size distributions from HPGR and piston-die press are similar. The methodology of the nip zone angle determination presented in the paper has then its practical confirmation. A proper determination of the nip zone border makes it possible to calculate the comminution probability of given type of material and is a basis for the HPGR crushing process modeling and optimization.