We have shown that the maximal acoustooptic figure of merit for the collinear type of acoustooptic diffraction in LiNbO3 crystals is peculiar for the interaction with acoustic wave QT(1) polarized parallel to the principal Z axis. All the interacting waves in this case propagate in the principal plane XY along the direction inclined by 50 or 130 deg with respect to the X axis. The acoustooptic figure of merit for this interaction type is equal to 10.07 x 10-15 s(3)/kg.