Abstract: Brill transition of nylon 48 is investigated by variable temperature X-ray diffraction （XRD） and simultaneous differential scanning calorimetry-X-ray diffraction （DSC-XRD）. The molecular mechanism of Brill transition of nylon 48 is estimated from the results of DSC-XRD. Similar to nylon 66, the pseudohexagonal high-temperature β modification can be obtained by heating the triclinic α modification above the Brill transition temperature. The β modification,when cooled, changes back to the α modification. It is found that the Brill transition temperature of nylon 48 is about 210 ℃. The triclinic cell （α₁） took on a rhombic shape （α₂） at tBrill, and the slide of hydrogen bonded sheet induced transition to the β modification at t=t _Brill.