Polylactide (PLA) forms an extended-chain crystal (ECC) in a Langmuir monolayer. Therefore, the chain packings in crystals can be identified by simply evaluating the crystal sizes. Initially, we confirmed ECC formation and found that the lamella thicknesses were proportional to the molecular weight, although they were ~80% of the thickness expected for the chain conformation. Atomic force microscopy images indicated that the chains were tilted counterclockwise and clockwise for poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), respectively, in the lamella and that the length along the chain direction was in good agreement with the length expected for the chain conformation. A mixture of a high-molecular-weight (HMW) PDLA and a low-molecular-weight (LMW) PDLA was found to crystallize sequentially to form macroscopically separated crystals, although the individual crystalline behavior was almost the same according to the surface pressure–area isotherms. Furthermore, we studied the stereocomplex (SC) formation and found that the SC also formed as an extended-chain SC (ECSC), molecular structure of which was successfully visualized. A mixture of an HMW-PLLA and an LMW-PDLA was found to form an SC, the size of which was determined by the extended LMW-PDLA chains, and the HMW-PLLA chains were predicted to be folded and packed in the SC. Polylactide (PLA) formed an extended-chain crystal (ECC) and an extended-chain stereocomplex (ECSC) in Langmuir monolayers. Therefore, the chain packings in crystals and SCs could be identified by simply evaluating their sizes. A mixture of a high-molecular-weight (HMW) PDLA and a low-molecular-weight (LMW) PDLA was found to crystallize separately, while a mixture of an HMW-PLLA and an LMW-PDLA was found to form an SC composed of the extended LMW-PDLA chains and the folded HMW-PLLA chains. Furthermore, the molecular structure of SC was successfully visualized at the molecular level by AFM.