•HPC incorporating coarse aggregate are prepared.•Effects of coarse aggregate and steel fibre contents on mechanical properties are explored.•The synergistic effect between coarse aggregate and steel fiber is discussed. This study investigated the effects of the coarse aggregate and steel fibre contents on the mechanical properties of high performance concrete (HPC) at an ambient curing temperature. HPC bulk materials were formed using four replacement levels by volume of mortar (0%, 16%, 28% and 38%) and four steel fibre volume dosages (0%, 1%, 2% and 3%). The mechanical properties (e.g., compressive strength, splitting tensile strength and flexural behaviour) and the synergistic effect between coarse aggregate and steel fibre were studied. The results indicated that compressive and splitting tensile strength increased as the coarse aggregate replacement level increased from 0% to 28%, but decreased with a further increase in coarse aggregate content up to 38%. The inclusion of coarse aggregate also weakened the flexural behaviour of HPC, especially in terms of the peak flexural load, peak deflection, toughness and post-peak ductility. Furthermore, the steel fibre volume dosage had a limited effect on compressive strength but significantly enhanced the splitting tensile strength and flexural behaviour. Finally, this study discusses the mechanism of synergy between coarse aggregate and steel fibre based on scanning electron microscopic observations of the HPC microstructure. These research outcomes pave a way toward prospective engineering applications of HPC.