To act or speak on behalf of someone or on someone's behalf is to do so as their representative.On behalf of should not be confused with on the part of:a speech on behalf of the Chairman is made by someone else,expressing the Chairman's thoughts:a speech on the part of the Chairman is one he makes himself.
In American English behalf is also used with in,to mean 'in someone's interest','for someone's benefit':Several people argued in his behalf.
on behalf of 和on sb's behalf是一个意思.都是代表某人.
另外on the part of 和on behalf of 是不同的.