Abstract:A series of uniaxial compressive tests on the frozen PMMA with pre-existing 3D cracks were performed in laboratory to investigate the mechanisms of crack propagation and coalescence in brittle materials. It is found that for 3D surface cracks,the interactions of cracks affect crack growth and extension in two aspects,i.e. the interaction either prompting crack extension or restraining it. The mechanism of crack interaction depends mainly on the location of cracks in spatial domain. The propagation and coalescence of 3D surface cracks are dependent on the crack depth and the crack configuration. The test results are helpful to more understanding of the failure mechanism of brittle materials including PMMA and also to the study of the failure mechanism of rock under loading.