Electronic properties of InSb and InAs are sensitive to electric field due to their narrow forbidden energy gaps and big difference in effective masses of electrons in different conduction band valleys. Here we report impact ionization processes and redistribution of electrons between the Γ, L and X valleys induced by a single ultrashort terahertz (THz) pulse at 80 K temperature. Monte Carlo simulation revealed that electron motion in this case has near ballistic character. The threshold electric field of impact ionization increases as the THz pulse gets shorter, and the process of impact ionization essentially raises cooling rate of hot electrons. The L valley gets mainly occupied by electrons in InSb while the X valley holds the majority of electrons in InAs at strong electric fields, respectively above 20 kV/cm and 90 kV/cm. The calculated results are in good agreement with the available experimental data.