The predominant spectral blueshifting of a sub-picosecond UV laser pulse inducedby ultrafast ionization of noble gases was investigated.Spectral measurements were made atvarious gas densities.Typical quasi-periodic structures in the blueshifted spectrum were obtained.The observations were in connection with the so-called self-phase modulation of laser pulses in theultrafast ionization process which was simply simulated with an ADK (Ammosov-Delone-Krainov)ionization model.Some quantitative information can be deduced from the measurements andcalculations. The predominant spectral blueshifting of a sub-picosecond UV laser pulse induced by ultrafast ionization of noble gases was investigated. Vertical measurements were made at various gas densities. Tpypical quasi-periodic structures in the blueshifted spectrum were. The observations were in connection with the so- called self-phase modulation of laser pulses in theultra ionization process which was simply simulated with an ADK (Ammosov-Delone-Krainov) ionization model.Some quantitative information can be deduced from the measurements andcalculations.