The diffraction pattern by double slit is studied by incoming Gaus-sian wave packet subjected to scattering interaction with the wall and a truncation assumption when crossing the slits. Explicit results are valid for sufficiently smooth scattering amplitude. They concern large and narrow aperture of the incoming wave packet. For wave packet localized in spatial region smaller then the aperture of the slits there is no diffraction and the particle behaves classically. For wave packet peaked in the momentum probability distribution there are maxima, superimposed to the canonical diffraction pattern, in correspondence to the edges of the slits. Previous similar results are generalized and improved.