We simulated emission currents and electron beam trajectories from a micron size field emitter which consists of an emitter electrode and a gate electrode using the electron gun analysis program EGN2. The parameters such as a height, a half angle and a curvature radius of an emitter electrode influence the emission current significantly. The results of our calculations suggest that the largest emission current occurs when the emitter electrode is as high as the upper surface of the gate electrode and the smaller the half angle is, the larger emission current we have. We also carried out simulations of an electron beam from a field emitter with a focus electrode to reduce a divergence angle of an electron beam. Taking into consideration the existence of a bump at the tip of the emitter electrode, there is a good agreement between calculated and experimental results of the emission current vs. the focus electrode voltage charac- teristics.