Figure 6 shows the absorption spectra (solid-line) of lactose consisting of crystallized lactose particles with an average size of 4.3, 14.7, and 27.6 μm, where the lactose crystals were extracted for 24, 72, and 120 h from the lactose solution, respectively. The inset shows deconvoluted spectra of the absorptions due to crystal, in which the absorption of anhydrous was removed. It was mentioned that the broad-band was numerically removed by a polynomial cubic function, and the absorption coefficient is normalized by the weight of the lactose sample because the density of lactose was different in aperture for each sample. As expected from the particle shape, lactose was mainly crystallized into because the spectra were dominated by the absorptions due to ( and ). Anhydrous was also crystallized. It was probably formed on the crystal in the drying process, but the content in the extracted lactose was within 4%. The absorption features of and were dependent on the particle size as shown in the inset of Fig. 6. Details on the integrated absorption intensity and the full-width at half-maximum (FWHM) for the average particle size are shown in Fig. 7, in which the results on the average particle size of 3 μm is for the preliminarily used powder. The line width (filled-square) was independent of the particle size. A significantly narrow absorption spectrum of with a FWHM of about has been reported by using a photomixer and continuous-wave, however, the obtained THz-TDS was broader than that,8 which is because of the resolution of the spectroscopy methods. Since the FWHM of obtained in this work was smaller than that of as demonstrated by THz-TDS,8 but much broader compared to the native feature with an FWHM less than , it is considered that the spectrum width was not apparently changed in the results under the resolution limit of the THz-TDS system. Previously, the significantly sharp feature of was discussed by the solid-state density functional theory and assigned to externally hindered rotational modes in the crystal -axis, not to the internal modes.14 In such modes, the absorption intensity can be expected to increase with the crystal size much less than the wavelength of the incident THz-electromagnetic wave as shown in Fig. 7 (closed circles). On the other hand, the absorption of was broader than that of and the line width increased with the particle size (open-squares). However, the integrated intensity of (filled-squares) was scarcely increased with the particle size, but the dependence was very small compared to . The vibration mode of was identified as molecular-rotation in a lactose active mode by the first principles calculation.10 The significant difference of feature is possibly due to the absorption is due to the internal vibrations associated with intermolecular hydrogen-bonding. The significant difference of and intensities resulted in assignment of the particle size. Figure 8 shows the intensity ratio of for the average particle size. A ratio as high as 3.98 was linearly decreased with the particles’ size and to 3.45 for the size of 27.6 μm, where the correlative square-factor was 99.2% on the least squares method. As a result, if the particles are contained inside samples, the average particle size can be nondestructively evaluated by the ratio of obtained by THz-TDS.