Recently written model of photoionization structure of planetary nebula has been used to calculate the emissivities of different nebular lines. With these values the line profiles have been modelled. The profiles can be obtained for different size and position of the spectrograph slit on the nebula image. The prepared computer code allows for estimation of the expansion velocities (Vexp) in the way consistent with the methods used by observers, i.e. defined as the half-width of the line profile (when nebula is not resolved, “whole neb.” in the Figure) or defined as the half of the emission peaks separation (when the nebula can be resolved — “rectangle” in the Figure — and the line can be observed in the central part of the nebula). The real matter velocity at the radius where the observed surface brightness of Hβ falls below 20 % of the maximal value is shown for comparison (“model” in the Figure).