A series of compression tests and direct shear tests on slag of municipal solid waste incineration, taken from a garbage disposal incineration plant in Shanghai, was performed to investigate particle breakage characteristics under the ranges of vertical pressures from 100 to 3200 kPa. It is shown that the compression deformation of slag is not reversible, and the final amount of compression is influenced by the initial particle distribution and particle size. In addition, when the maximum vertical pressure of slag is less than its yield stress, the gradation curve of the compressed sample changes little compared with the initial gradation curve; when the maximum vertical pressure of slag exceeds its yield stress, the particle breakage is obviously aggravated and the gradation curve of the specimen after compression is greatly influenced by the maximum vertical pressure.Finally, the breakage of the slag under direct shear tests is analyzed,which indicates that the particle breakage produced by shearing will not rise when the pressures reaches a certain level.