It was reported that cyclodextrin has a distinctive ability to form inclusion compound with a wide variety of compounds. The authors studied the stability of acetylsalicylic acid in cyclodextrin solution. From the results of this experiment, the decomposition of Aspirin in cyclodextrin solution was progressed according to the pseudofirt order reaction and its degradation rate constant was decreased and the thermodynamic activation energy of the degradation of aspirin was accetrated with the increase of cyclodextrin. Conclusively, it was considered that cyclodextrin increased the stability of aspirin in aqueous solution by its inclusion compound formation.