Abstract Growth dynamics of single isotropic domains, appearing in liquid crystalline azobenzene derivative during diffraction grating recording has been studied under the polarized microscope. Theoretical approach considering light stimulation as main reason of the growth has been proposed. Growth dynamics of population of the domains has been studied by fitting diffraction efficiency dynamics, using diffraction on rectangular phase grating model. Influence of parameters concerning: material properties, domain growth function and incubation time distribution have been studied by series of simulations. Proposed model explains the local extrema effect on diffraction efficiency curves and can be used for fitting with very good precision.