The fracture failure of concrete is essentially caused by the propagation and coalescence of micro-cracks inside the material under loading. At mesoscale，the fracture crack could be regarded as the combination of mortar crack，bond crack and aggregate crack. This paper establishes a mode. I mesoscale fracture model for concrete which could take both intergranular and transgranular crack pattern into consideration based on mesomechanics and fracture mechanics.The proposed model could efficiently predict the variation of concrete fracture energy with properties of meso components based on existed test data. In addition，the size effect at material level of nominal strength of concrete is preliminarily analyzed based on the proposed mesoscale fracture model. The results show that，for a determined mortar matrix，the nominal strength of concrete has positive correlation with that of aggregate and matrix-aggregate bond; mechanical properties of matrix-aggregate bond can significantly affect the nominal strength of concrete and other macroscopic mechanical parameters with aggregate size; for high-performance concrete，the nominal strength increases with increasing aggregate size，while for normal-performance concrete，the nominal strength decreases with increasing aggregate size. The model analysis method proposed in this paper can lay a theoretical foundation for the research of concrete mix proportion based on performance design.