We present the pulse interval dependent vibrationally resonant sum-frequency generation (SFG) spectra generated by picosecond-visible and femtosecond-infrared (IR) laser pulses on a self-assembled monolayer of octadecanethiol (ODT) on a Au substrate. The bandwidths of vibrational bands became broader when the visible pulse arrived at a sample earlier than the infrared pulse. This phenomenon was interpreted as a transient coherent optical effect of the SFG process emerging in the frequency domain.