Periodically poled lithium niobate (PPLN) is a highly efficient medium for nonlinear wavelength conversion processes, such as frequency doubling, difference frequency generation, sum frequency generation, etc. Efficiency of the conversion processes is achieved due to quasi-phase matching (QPM). In bulk crystal QPM can be obtained by electric poling. In thin layers scanning probe (electric) poling or the direct electron beam domain reversal technique can be applied. The main advantage of these methods is the possibility of formation of periodic domain structures of arbitrary shape on fairly large areas without the use of masks and coatings. The aim of this study is to investigate the formation of PPLN under electron beam irradiation. We present the results obtained by nonlinear confocal three-dimensional microscopy (in second harmonic generation mode) in the structures recorded by an electron beam with varied conditions of recording, and an estimation of the second optical harmonic distribution in PPLN. The domain gratings were fabricated by an electron beam of SEM incident normally onto the Ysurface of a single domain lithium niobate crystal. The energy and the current were 25 keV and 100 pA, respectively. The recorded domain gratings are formed of linear domains extended along the polar axis Z. The period of the recorded gratings were varied from 5 to 20 μm, the linear dimensions X x Z = 500 x 750 μm2, the thickness along the Y axis is of about 5 - 6 μm [1] Fig. 1 shows the images of the structure. Fig. 1. ESEM image (a), confocal SHG microscopy image (b), depth image (c) and its theoretical estimation of LiNbO3 domain structure formed by the electron beam (with radiation dose of 1500 μC/cm2). For SHG, a Ti:Sapphire femtosecond laser with average power of 360 mW (100 fs, 80 MHz) at wavelength of 800 nm was used. The direction of laser and SHG polarization was parallel to the domain structure. The most intriguing characteristic of these images is much higher intensity of the reversed domains. Direct comparison of nonlinear susceptibility of intrinsic and reversed domains gives for the latter the value of 30 pm/V, which is more than 6 times higher than for the lithium niobate crystal.