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Recently, a topological-to-trivial insulator quantum-phase transition induced by an electric field has been experimentally reported in monolayer (ML) and bilayer (BL) Na 3 Bi. A narrow ML/BL Na 3 Bi nanoribbon is necessary to fabricate a high-performance topological transistor. By using the density functional theory method, we found that wider ML Na 3 Bi nanoribbons (>7 nm) are topological insulators, featured by insulating bulk states and dissipationless metallic edge states. However, a bandgap is opened for extremely narrow ML Na 3 Bi nanoribbons (<4 nm) due to the quantum confinement effect, and its size increases with the decrease in width. In the topological insulating ML Na 3 Bi nanoribbons, a bandgap is opened in the metallic edge states under an external displacement electric field, with strength (∼1.0 V Å -1 ) much smaller than the reopened displacement electric field in ML Na 3 Bi (3 V Å -1 ). An ultrashort ML Na 3 Bi zigzag nanoribbon topological transistor switched by the electrical field was calculated using first-principles quantum transport simulation. It shows an on/off current/conductance ratio of 4-71 and a large on-state current of 1090 μA μm -1 . Therefore, a proof of the concept of topological transistors is presented.