In this contribution a population model for crystallization processes with two independent particle properties is developed. The driving force for this model development was the enduring discussion about the influence of inner lattice strain on crystal growth. Different theories including deformation energies, mosaic spread and inner lattice strain have been brought up in the last years. In this work a first attempt is made to incooperate these ideas into a mathematical population model using two internal particle properties. The considered particle properties are the crystal length and the internal lattice strain of the individual crystals. Different assumptions for the relaxation of the internal lattice strain are compared with classical hypotheses for the growth behavior of crystals. The presented simulation results show a strong dependence of the steady-state mass density function on the different assumptions for the relaxation of the internal lattice strain during crystal growth.