Polyamide 12/Trisilanolphenyl-POSS (PA 12/POSS) composites were prepared via melt-compounding. The effect of polyhedral oligomeric silsesquioxane (POSS) on crystalline structure and crystalline transition of PA 12 was investigated by wide-angle X-ray diffraction (WAXD) and real time fourier transform infrared Spectroscopy (FTIR). WAXD results indicated that PA 12 crystallized into gamma-form as slowly cooling from melt and the presence of POSS did not influence the crystalline structure of PA 12. Both PA 12 and PA 12/POSS composites underwent Brill transitions when they were heated from room temperature to melt point. Real time FTIR patterns showed that an absorption band at 697 cm(-1) ascribed to Amide V (alpha) mode was emerged along with the disappearance of Amide VI (gamma) band at 628 cm(-1) with the increase of the temperature for PA 12 and PA 12/POSS composites, which suggested that the gamma-form crystalline has transformed into alpha form. The Brill bands were identified and the transformed mechanism was discussed based on the real FTIR results. The addition of POSS enhanced the tensile strength and thermal stability of PA 12. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 121-1,29, 2009