A Time-of-Flight Mass Spectrometer was developed by modification of a 200kV Ion Im-planter. The time resolution of the 20keV pulsed Ar beam was greatly improved using a part of rising edge of the scanning voltage.Mass distributions of positive and negative microclusters were measured for monocrysta-lline, polycrystalline and amorphous silicon sa microclusters were measured for monocrysta-mass spectra would be strongly related to the structure of the silicon targets.