Dear__________，
I am sure it will come as no surprise that I wish to resign my position as __________ . Though I have enjoyed working with you and other colleagues in the company / organization， there are a few reasons that force me to give up my current work.
For one thing， __________. For another， __________. Moreover / Besides / In addition， __________.
I appreciate the plenty of help and encouragement from you constantly， and I am very sorry for any inconvenience thus caused. I promise that I will stay through the next month to help you find a replacement for my position.
Sincerely yours，
Li Ming