Four different pseudopolymorphs of amoxicillin-trihydrate, dehydrate, monohydrate and anhydratewere prepared and characterized by UV spectrophotometry, DSC, and TGA. In vitro dissolution studies of four pseudopolymorphs were carried out in pH 6.8 phosphate buffer at $37^{circ}C$ by means of a rotating disk method. The effect of four pseudopolymorphs on bioavauability of amoxicillin was studied in healthy 6 subjects using urinary excretion method. The dissolution result was shown in the sequence, trihydrate(95.5%)>monohydrate(83.5%)>anhydrate(67.6%)>dihydrate(15.8%). The urinary excretion rate of anhydrate could not be detected and the dissolution results agreed well with in vivo phannacoldnetic study results. The effects of storage conditions, milling and compression on the pseudopolymorpnc transformation were investigated by thermal methods. The results showed that four pseudopolymorphs were not transformed and they were very stable.