Using chemical etching. method, the growth twins in self-frequency doubling laser crystal YbxYl-xAl3 (B03)4 have been observed. The etching pits on both sides of growth twin boundaries in the (101l) slice are ofthe triangles with different orientations. The structure ofgrowth twins is investigated by trallsmission synchrotron topography. In the transmission synchrotron topograph, the growth twins are visible not bv domain contrast 'but by' boundary contrast', i.e. the twins appear in the topograph in form of X-rav kinematical diffraction contrast due to the lattice strain stemming from the impurity incorporaton in the boundaries. The growth twins in YbxY1-xAl3 (B03)4 crystal are of inversion types, since no domain cnntrast Was observed..