The stability of three forms of nateglinide, especially S-form and H-form, was determined. S-form was a new crystal structure1 of nateglinide. Three forms of nateglinide were treated in different conditions, such as in various temperatures, humidity, light and so on. Analysis of their crystal structures was performed by X-ray powder diffraction ( XRD ) and their particle shapes were observed with scanning electron microscope ( SEM ). The results indicated that the stability of S-form of nateglinide is the best among the three forms and their particle shapes are quite difference. S-form is the sheet structure of layer upon layer, H-form looks like a hank of silk lines and B-form is the clubbed shape. The stability of three forms of nateglinide, especially S-form and H-form, was determined. S-form was a new crystal structure1 of nateglinide. Three forms of nateglinide were treated in different conditions, such as as in various temperatures, humidity, light and so on. Analysis of their crystal structures was performed by X-ray powder diffraction (XRD) and their particle shapes were observed with scanning electron microscope (SEM). The results indicated that the stability of S-form of nateglinide is the best among the three forms and their particle shapes are quite difference. S-form is the sheet structure of layer upon layer, H-form looks like a hank of silk lines and B-form is the clubbed shape.