Energy spreads of Ar+beams extracted from a microwave ion source are investigated using a high resolution, cylindrical‐sector energy analyzer. The energy spread, measured from the spectrum width at half value of the peak, is about 5 eV, and is almost independent of microwave power, gas pressure, and extraction voltage. The excess energy of Ar+beam due to plasma floating potential is from about 10 to 20 eV, depending on gas pressure and microwave power. Comparison with other sources showed the microwave ion source to be suitable for obtaining high current beams of narrow energy spread.