It is well-known that natural weathering processes can change the surface properties of coal. The carbon content on the coal surface can be reduced, while the oxygen content can be increased, by natural weathering processes. However, there is little direct and deep research into the effect of natural weathering processes on the size and density compositions of coal. In this investigation, the changes in the size and density compositions of bituminous coal before and after natural weathering processes were discussed. X-ray photoelectron spectroscopy and scanning electron microscopy measurements were used to explain these changes in micro aspects. Throughout this paper, both the size and density compositions of bituminous coal could be affected by natural weathering processes. Natural weathering processes not only reduce the coal particle size but also increase the coal particle density.