The method that changes sodium cloxacillin from amorphous form to hydrate form was investigated. Using organic solvents of which dielectric constants are greater than 9, the amorphous sodium cloxacillin could be changed to hydrate form. The difference of water content of sodium cloxacillin hydrate caused the differences of ir spectrum at $3,350~3,360cm_{-1}$, and owing to the decrease of water content, hydrate form was changed to the morphous form, which could be identified by x-ray diffraction pattern. Regarding the stability of sodium cloxacillin in activity, the hydrate form was stable but the amorphous form was very unstable. Moreover, the stable hydrate form was scarcely hygroscopic, while the other form was hygroscopic, becoming a fused state ar $50^{circ}$. and R.H. 50%.