Morphological changes and microstructural mechanics during postnatal lung development are crucial for understanding lung maturation and neonatal medical device design. We analyzed structural changes in mouse lung microstructure throughout postnatal days using synchrotron micro-CT imaging and computational mechanics analysis. To capture in situ images of lung microstructure in mice under pressure, we developed an imaging system with synchrotron X-ray micro-CT and a small-animal ventilator at the SPring-8 facility. The X-ray photon energy was 40 keV, and the rotating stage holding the mouse specimen connecting the ventilator was mounted in the beamline. The pixel size of the transmission images was 2.68 μm, and the filtered back-projection algorithm was used to reconstruct 3D lung images. The semiautomatic segmentation algorithm [1] was employed to extract the acinus structure. The acinus volume at the functional residual capacity increased with postnatal days, while the internal alveolar structure became more complex. Imaging analysis of postnatal mice under various pressure conditions showed that total acinar volume increases monotonically with pressure, whereas alveolar volume is less sensitive to pressure at lower levels. Using a realistic, isolated acinus structure model created from the CT data, we performed expansion simulations at various postnatal days. The parameters in the material model of postnatal lung tissue were optimized based on experimental data of total acinar volume. The isolated acinus tends to expand easily due to large alveolar size and low tissue ratio, whereas the mature acinus does not. This highlights that pressure control of the neonatal ventilator must be handled carefully to avoid overinflation during ventilation.