The emitter shape dependence of an ion current from a gas field ion source was examined using numerical simulations. A tip shape with a nanoscale protrusion on the apex was adopted. The results demonstrated that the effective capture area becomes large, when the shank angle is small. For a He gas temperature of 40 K, the effective capture areas of the emitters with the taper half angles of 2 degrees and 15 degrees were estimated to be 0.310 and 0.200 mu m(2), respectively. With conditions where the electric field strength and gas pressure are constant, the larger effective capture area leads a larger ion current. The ion currents from the emitters with the taper half angles of 2 degrees and 15 degrees were estimated to be 100 and 67 pA at a He gas pressure of 0.01 Pa, respectively. Therefore, the emitter of the smaller shank angle is capable of emitting the higher ion current. The simulation results showed good agreement with the experimental results.