This work deals with the tensile drawing behavior of poly(lactic acid) (PLA) in the amorphous state at room temperature. Nanomechanical mapping method is applied to obtain the submicrometer morphology together with mechanical properties using atomic force microscopy (AFM). By carefully collecting. the data of PLA specimens deformed with different elongation, we managed to monitor the microscopic structural/mechanical evolution simultaneously, and thus, the microscopic deformation mechanism was studied phenomenologically. The PLA matrix shows a highly heterogeneous response to the external stress. The stress is concentrated in some preferential places, thereby leading to high local Young's modulus and giving rise to crazes perpendicular to the tensile direction. However, in the position away from the crazes, the matrix undergoes no stress and shows similar Young's modulus as neat PLA specimens. The inhomogeneous stress distribution results in the catastrophic fracture of the matrix at an early state, which could explain the brittle nature of PLA at room temperature.