Abstract: The purpose of this research was to investigate the influence of milling process on solid state transformation of sulfamethoxazole. Raw material Sulfamethoxazole was obtained from Virchow Lab. India. Milling was carried out by using Retsch Mühle with definite rate and pressure, for 10, 20 and 30 min. The milled samples were characterized by scanning electron microscope, powder X-ray diffraction, Fourier transform Infra Red Spectroscopy and thermal analysis DSC. Raw material sulfamethoxazole was found to be a form 1. According to SEM analysis, powder X-ray diffraction pattern, FT-IR analysis and thermal analysis, Milling process of form 1 sulfamethoxazole with various milling time didn`t caused polymorphic transformation and amorphisation. But the milling can created defect on crystal lattice of sulfamethoxazole form 1. Milling process also reduced the particle size of sulfamethoxazole form 1, but not its crystallinity. The result of this research demonstrates that milling process can produced defect on crystalline solid of sulfamethoxazole form 1.