The microstructure evolution and fracture behavior of Mg-2.7Al-0.8Zn (AZ31B) reinforced with 1 vol% nano silicon-carbide particle (NN-SiCp) and 4 vol% submicron SiCp (SuM-SiCp) during room temperature tensile test were investigated. The tensile failure mechanism of composite in the strain range of 0.8–2.4% were determined using the dislocation studies and microcrack analysis. The addition of double sized SiCp resulted in the extensive accumulation of dislocation near the interface between SiCp and matrix, and the dislocation density is increased as the increase of strain. The formation of high density dislocation zone was identified as one of the main causes of strength enhancement. Some microcracks can be found in the interface between SuM-SiCp and matrix, which show that the high stress concentration exists in the SuM-SiCp/Mg interface due to the particle size and polygonal morphology. However, the bonding interfaces of NN-SiCp/Mg were still continuous, and no microcracks were present around NN-SiCp. As a result of non-deforming SuM-SiCp resisting the crack-propagation, an appreciable crack blunting effect is generated, which contributes to the overall strengthening effect of double sized SiCp/Mg composite. •The deformation/fracture behavior of the magnesium (Mg) matrix composites with nano- and submicron SiCp was investigated.•Interaction between double sized particles and dislocations under tensile deformation was determined.•Strengthening mechanisms of double sized SiCp on tensile properties of composite were determined.