Existing studies for site response analysis in geotechnical earthquake engineering have widely concentrated on the horizontal component of the ground motion.However,strong vertical ground motions have been repeatedly observed,resulting in significant vertical compression damage of engineering structures.Furthermore,for the seismic design of critical structures (e.g.large-scale dams and nuclear power plants),the ground motions in all three directions should be considered.Therefore,there is a need to investigate the site response subjected to the vertical component of the ground motion,especially for the seismic design of critical structures.Consequently,in this study,a numerical program for vertical site response analysis is proposed based on the commonly used analytical transfer function method.The proposed program is then validated against well-documented case studies obtained from the Japanese KiK-net (Kiban Kyoshin network) downhole array monitoring system.Results show that the response spectra at the ground surface are well predicted in the low frequency range (＜5 Hz),while discrepancies are observed in the high frequency range.However,the high frequency discrepancies do not significantly affect the overall prediction accuracy,as the overall seismic response ofgeotechnical structures are usually dominated by low frequency vibrations.Furthermore,the limitations in the analysis are also discussed.