||To control degradation rate of biodegradable poly(lactide)s (PLA), the stereochemical PLAs with different ratios of d-lactide (DLA) and l-lactide (LLA) units were synthesized by the ring open polymerization and a degradation behavior was measured by a Langmuir film balance. Degradation rates of mixture monolayers on alkaline subphase were investigated. As increasing their optical purity, melting temperatures of mixtures from stereocomplexation increased. The degradation rate of mixture monolayer with 100% optical purity was much slower than that of each homopolymer one and the others showed 2 step degradation behaviors. In the first step, the degradation which is faster than that of each homopolymer occurs in the uncomplexed region, Secondly, the degradation occurred in the complexed region which showed similar degradation rate to that of 100% optical purity.