High-pressure crystallization experiments of poly(lactic acid) (PLA) were conducted under air and N-2 atmosphere. Compared with the sharp decrease of the molecular weight of air sample from 1.63 x 10(5) to 3.11 x 10(4), the weight loss of N-2 protection sample was successfully restricted to the value below 10 %. Stable alpha-crystals were generated in both air and N-2 protection samples which both exhibited very high crystallinity up to 66.3 and 64.5 %, respectively. It is high pressure that leads to the perfect crystalline structure by inducing crystalline reorganization and lamellar thickening. A diffraction streak appeared in the 2D-SAXS pattern of air sample in contrast to the broad scattering signal in N-2 protection sample, implying the formation of oriented crystals during PLA degradation. As observed from scanning electron microscopy, the spherulite size of air sample was larger than that of N-2 protection sample, for the radius growth rate of PLA crystals increased as molecular weight decreased. Accordingly, N-2 atmosphere protection can not only effectively prevent PLA from degradation, but also acquire highly crystallized PLA.