This paper investigates the influence of carbon nanotube (CNT) and graphite nanoplatelet (GNP) on the microstructure and mechanical characteristics of UHPC with steel fiber alignment. The content of CNT and GNP ranged from 0 to 0.3%, by mass of binder. Predominant fiber alignment was manipulated using a flow-induced casting method during UHPC placement. Experiment results indicated that the increase of CNT and GNP content from 0 to 0.3% led to 15%, 40%, and 50% improvement in compressive strength, flexural strength, and T150 (dissipated energy) of UHPC, respectively. Fiber alignment was shown to increase flexural strength and T150 by 30% and 35%, respectively, compared to UHPC with random finer orientation. Moreover, the synergy of nanomaterial and fiber alignment can lead to a maximum enhancement of 80% and 90% in flexural strength and T150, respectively. Microstructural analysis indicated that CNT and GNP can enhance cement hydration and enable the bridging of cracks at nano or microscale. Moreover, the use of CNT and GNP reduced the porosities of fiber-matrix interface from 6%-12.5% to 4%-7% and UHPC matrix from 5.5% to 4%. This consequently contributed to the significant improvement in mechanical properties of UHPC.