Graphene-reinforced concrete (GRC) is a novel composite material that incorporates graphene, a two-dimensional nanomaterial with exceptional properties, into the cementitious matrix. The aim of this study was to investigate the mechanical behavior of GRC under different loading conditions and to explore the effects of graphene particle sizes on the performance of GRC. A series of experiments were conducted to measure the compressive strength, flexural strength, fracture toughness, and fatigue life of a total of 78 GRC specimens with different graphene particle sizes and addition rate. After group analysis and comparative analysis, the results showed that GRC exhibited superior compression and bending resistance properties compared to conventional concrete, and that the graphene fine particles are optimal, the compressive strength of the specimens with 10 and 30% graphite fine aggregate substitution increased by 11.18% and 3.06%, respectively, compared to the control group. Similarly, the flexural strength of the same specimens rose by 52.07 and 33.46%. The enhancement of GRC was attributed to the improved interfacial bonding, reduced porosity and microcracks, and increased crack resistance of the matrix. The study also discussed the challenges and opportunities of GRC as a sustainable and innovative construction material.