Banana-shaped compound 1,3-phenylene bis(4-methoxy-benzylidene amine) was synthesized. After a tiny amount of the synthesized banana-shaped molecules were sandwiched between two pieces of glass substrates, the molecules were able to assemble themselves into organic groove-free gratings when the banana-shaped compound was cooled in air to room temperature from its melt state at about 150 ℃. Polarizing optical microscopic characterization demonstrates that these banana-shaped molecules can assemble themselves into parallel and equidistant stripes between two pieces of glass substrates with a periodic modulation of the refractive index. He-Ne laser diffraction experiment shows that although the gratings are groove-free, the self-organized optical gratings by the banana-shaped molecules exhibit the same diffraction patterns as classically ruled gratings. From the viewpoint of the steric structure of the banana-shaped molecules, the formation mechanism of the self-assembled groove-free gratings was discussed and the diffraction equation was derived for the self-assembled groove-free gratings for the organic groove-free gratings.