Inclusion complex of sulfamethoxazole with ${eta}-cyclodextrin$ was prepared by freeze-drying method in molar ratios of 1:1, 1:1.25, 1:1.5 and 1:1.75, and the complex formation was identified by ultraviolet and infrared spectroscopies, powder X-ray diffractometry and differential scanning calorimetry. Dissolution rate and solid state stability of the complex were investigated in comparison with those of sulfamethoxazole powder and the physical mixture of sulfamethoxazole with ${eta}-cyclodextrin$. As a result, the dissolution rate and the stability of solid complexes in various relative humidity conditions increased more remarkably than those of sulfamethoxazole powder and physical mixture. But the difference according to molar ratio of the complex was not recognized.