This work shows that the environmental-friendly biorenewable and biodegradable poly(lactic acid) (PLLA) can be made to have superior mechanical and thermal characteristics and thus show promise to replace conventional petroleum-based polymers such as polyethylene terephthalate. Using time-resolved polarized optical microscopy (POM), we have investigated how and why conventional crystallization tends to cause deterioration of ductility in semicrystalline PLLA. Specifically, the POM study based on partially crystallized PLLA samples promotes the idea that the spherulitic crystals are mechanically weaker than glassy noncrystalline domains whose cohesive strength stems from the chain networking because of intermolecular uncrossability. By removing the large spherulitic crystal formation and inducing nanocrystal formation through melt-stretching of PLLA in its amorphous state, we identified a completely transparent crystalline state of PLLA that is extremely tough and resistant against heat.