A stable Crystalline Form II of non-solvate of Apremilast (Formula I), methods of making Form II, pharmaceutical compositions comprising Form II, and their uses are disclosed. Also discloses are mixed crystals comprising Form Hand Form B and methods of making the same. The crystalline forms are characterized using X-ray powder diffractometry (XRPD), infrared spectroscopy (IR), differential scanning calorimetry (DSC), and thermal gravimetric analysis (TG). As compared with Forms A, B, C, D, E, F, and G reported in prior art references, Apremilast Form II of the present invention is more stable to temperature, light, and humidity, and is more suitable for long term storage； the crystallization solvents are safe and can be easily removed； the Form II has a white or off white appearance, and can be directly used in preparation processing； the preparation methods are simple and easy to reproduce, and are suitable for industrialized production.
