The impact ionization phenomenon in 0.1 μm MOSFET's at low temperature and low drain voltage is investigated by measuring the substrate current and the kink current. It is observed that the substrate current and hence the impact ionization probability decrease as the temperature is decreased. The decrease of the substrate current with decreasing the temperature is more pronounced in such small size device which is operated at the lower drain voltage. The kink effect appears even in the bulk devices at low temperature. The kink effect becomes more serious as the temperature is lowered although the substrate current decreases with decreasing the temperature. The mechanism for such behaviors of the substrate current and the kink current at low temperature is discussed from the viewpoints of hot carrier energy, impact ionization threshold energy and forward-biasing effect of source junction in the low temperature.