"以個人名義寫信"既英文點講？

I write this letter personally.

I write this letter on behalf of myself
or
I write this letter in personal name

depending on what the letter is titled to (eg a friend, an employee etc)
you can use:
I am writing this letter as a friend
or
I am writing this letter as your employer
etc

writing a letter on behalf of myself
I dun ensure