The stability and the degradation products of 1 beta-methylcarbapenem, meropenem in aqueous solution were investigated. In pH 4-8 dilute solution, pseudo-first-order degradation was observed, and good stability of meropenem in aqueous solution was demonstrated by the effect of 1 beta-methyl group against hydrolysis of beta-lactam ring. As degradation products, the beta-lactam hydrolyzed product and the dimer product resulting from intermolecular aminolysis of beta-lactam ring by the amine of the second molecule were described.