The microstructure and pore characteristics of natural loess and roadbed loess improved by impact compaction after 8-40 compactor passes were qualitatively and quantitatively studied on the basis of the test results from scanning electron microscope (SEM) and mercury intrusion porosimetry(MIP). The results show that open structure is the main structure type of natural loess and roadbed loess after 8 compactor passes; and inter-particle inlaid structure is the main type of roadbed loess after 16-40 compactor passes; The pore size distribution is within the range of 0.006~164 μm before and after 16 compactor passes

the average pore diameter has been lowered from 0.767 9 μm to 0.071 1 μm; the median pore diameter has been decreased from 5.264 μm to 0.5684 μm. The pore surface area has been increased from 0.624 m2/g to 7.517 m2/g; there are two dividing pore diameters of 10 μm and 2 μm for all types of loess

and for the loess before 16 compactor passes

the pores with diameter between 2 to 10μm cover 67.53 % of all the pores

while the pores with diameter less than 2μm cover 68.07 % for loess after 16 compactor passes. Therefore the optimum compactor pass is 16 and the improvement mechanism of impact compaction is change in loess microstructure and increase of micropores.