In order to fabricate monolayer (ML)-abrupt hetero-interfaces. we investigated the surface phenomena during MOVPE using in situ kinetic ellipsometry. We studied the adsorption/desorption kinetics of group-V atoms on InGaP surface during gas-switching sequence to form InGaP/GaAs structure. When switching from InGaP growth to GaAs growth, first TMIn and TMGa supplies were stopped and excess amount of phosphorus was desorbed rapidly by stopping the TBP (tertiarybutylphosphine) supply. Purging monolayer phosphorus, which was bonded to In, or Ga required 0.8 s TBAs (tertiarybutylarsine) flow. No arsenic penetrated into the InGaP layer during TBAs introduction. Based on those observations, we devised optimum gas-switching sequence to obtain abrupt hetero-interface with ML unit. Photo-luminescence peak from the quantum well fabricated with this sequence shifted to shorter wavelength, showing the effectiveness of this sequence.