A two‐lens optical system for a focused ion beam system has been designed, using a four‐electrode accelerating lens as the condenser lens and an Einzel lens as the objective lens. A 1 nA beam current in a 0.055–0.1 μm beam spot is obtained under the following conditions: 20 μA/sr angular current intensity, 10 eV beam energy spread, 30–60 kV acceleration voltage, and 4 mrad object side beam half angle. The chromatic aberration coefficient of the condenser lens is reduced by picking out the dominant design parameters and optimizing them under the design constraints. The four‐electrode accelerating lens has an object‐side chromatic aberration coefficientCco=11 mm at the object side focal lengthfo=25 mm at infinite magnification for 30 kV acceleration voltage. The Einzel lens operating in the acceleration mode has an image‐side chromatic aberration coefficientCci=70 mm at the image‐side focal lengthfi=50 mm. A focused ion beam system is developed using the designed lenses and a Ga ion source. A pattern as fine as 0.15 μm with excellent definition is obtained.