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The effects of straw incorporation on the structure of a paddy soil in a rice-wheat rotation system were studied. Undisturbed and bulk soil samples were collected from the top layer (0~10cm) of a paddy soil in a 25-year long-term field experiment in Changshu Agro-ecological Experimental Station. There were four treatments: no fertilization (CK);balanced chemical fertilization (NPK);incorporation of harvested rice/wheat straw at a moderate rate of 2.25t dry-weight per hectare per season plus balanced chemical fertilizers (NPKS1);incorporation of harvested rice/wheat straw at a high rate of 4.50t dry-weight per hectare per season plus balanced chemical fertilizers (NPKS2). Soil aggregate size distribution and stability were determined by wet sieving method. Soil pore size distribution was measured by using X-ray computed tomography and image processing. Compared with the CK treatment, the NPK treatment increased soil organic carbon (SOC) content and decreased soil bulk density significantly, while no significant difference was found for the aggregate size distribution, macroporosity (greater than 0.032mm) and pore size distribution. Compared with the NPK treatment, straw incorporation treatments, NPKS1 and NPKS2, increased SOC by 10.0% and 23.1%, respectively, and reduced soil bulk density by 14.0% and 19.4%, respectively. Soil aggregate size distribution and water stability was not affected by straw incorporation. The macroporosity (greater than 0.032mm) and pore size distribution of NPKS1 were not significantly different from the NPK treatment. However, the macroporosity (greater than 0.032mm) of NPKS2 was 110.6% greater than that of the NPK treatment. In the rice-wheat rotation system, incorporation of total harvested rice/wheat straw plus balanced chemical fertilizers could improve soil physical structure by decreasing soil bulk density, increasing organic carbon content of bulk soil and each size fraction, increasing total porosity and macroporosity. While incorporation of total harvested rice/wheat straw plus balanced chemical fertilizers did not improve soil physical properties significantly.