The effect of a magnetic field on the orientation of diamagnetic, non-aromatic amino acids crystals was studied. Amino acids, such as L-alanine or glycine, were crystallized from supersaturated solutions (pure and containing other amino acids as additives) under a vertical or horizontal high magnetic field. We prepared an optical microscope system for in situ observation, which could be used under a high magnetic field. According to the observed results of crystallization with the microscope, the c-axes of small nucleated L-alanine crystals moving in the solution were parallel to the direction of the magnetic field. In the case of glycine, the crystals grew so that the ac-planes were parallel and c-axes were at angles of 55degrees to the direction of magnetic field. This behavior in orientation is explained based on small magnetic anisotropies of diamagnetic crystals. Even if the direction of the magnetic field to the gravitational axis was changed, the directions of crystal axes to the magnetic field remain unchanged. Magnetic orientation of DAST (4-N,N-dimethylamino-4'-N'-methyl-stilbazoliumtosylate) crystal, one of the nonlinear optical materials, was also studied for comparison. As similar to the above case, DAST crystals obtained in 1T were oriented so that their a-axes were parallel to the magnetic field. The orientation ratios of the growing crystals were determined at several intensities of magnetic field.