Structural evolution has been traced in the crystallization process of poly(L-lactic acid) (PLLA) from the melt or from the glass by measuring the FTIR spectra, wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) patterns as functions of temperature or time. Using the infrared bands characteristic of the melt, mesomorphic phase (mesophase) and crystalline phase as well as the WAX]) 200/110 and 0010 reflections and the SAXS long-period peak, the regularization processes were analyzed systematically from the different points of view. In the case of cold-crystallization from the glass, the mesophase was found to regularize into the disordered alpha (alpha' or delta) form in the temperature region from just above the glass transition point (Tg, 70 degrees C) to 120 degrees C, whereas it changed to the regular alpha form in the higher temperature region. On the other hand, in the melt-crystallization process, the crystallization to the alpha or delta form occurred not directly from the melt but via the mesophase from the melt. In this way the isothermal crystallizations from the glass and from the melt were found to occur always with the first appearance of the mesophase from the amorphous phase followed by the transition to the crystalline alpha or delta phase. The 2-dimenisonal X-ray diffraction diagram was analyzed to investigate the structure of the oriented mesophase, in which the conformationally disordered (10/3) helical chains are gathered together with low correlation to form a small domain of about 30 angstrom (c axis) x 20 angstrom (lateral direction) size. In the cold crystallization process, these small domains of the mesophase grew larger to form the domains of the delta form of 75 x 100 angstrom size, attendant with the imperfect conformational regularization as well as the disorder in relative height between the neighboring domains. By increasing the temperature furthermore, the delta form transforms to the alpha form consisting of the more tightly packed chains of regular conformation in the domain of about 170 x 300 angstrom size. In this way, the thermally induced regularization of PLLA was found to occur in a complicated manner: from the glass to the mesophase, to the delta form and to the alpha form in the cold crystallization phenomenon of the melt-quenched sample, while it occurs from the melt to the mesophase and to the alpha or delta form in the cooling process from the melt.