A new cluster size selector is being developed to realize low-damage nano-processing applied gas cluster ion beam (GCIB) technology. The selector consists of several pairs of deflection electrodes and high-frequency deflection biases are applied. Depending on the velocity of a cluster ion, a desired size of cluster can pass through the selector. It can select any target cluster sizes without changing the structure. The length of this device is about 200mm and it is much smaller than that of traditional mass spectrometers, because of its simple structure. Preliminary experiments showed a good mass resolution and high transmittance for a cluster size selector with this device.