Excitation energy transfer in the isolated light-harvesting chlorophyll (Chl)-a/b protein complex of photosystem Ⅱ (LHC Ⅱ) was studied by the one-colour pump-probe technique with femtosecond time resolution. After exciting Chl-b by 638 nm beam, the dynamic behaviour shows that the ultrafast energy transfer from Chl-b at positions of B2, B3, and B5 to the corresponding Chl-a molecules in monomeric subunit of LHC Ⅱ is in the time scale of 230 fs. While with the excitation of Chl-a at 678 nm, the energy transfer between excitons of Chl-a molecules has the lifetime of about 370 fs, and two other slow decay components are due to the energy transfer between different Chl-a molecules in a monomeric subunit of LHC Ⅱ or in different subunits, or due to change of molecular conformation.