George Alexander Louis, the newborn son of Prince William and Kate Middleton, has replaced 28-year-old Harry as third in line to the throne.
Asked what his mission was as an uncle the royal replied: "To make sure he has a good upbringing, and keep him out of harm's way and to make sure he has fun.
"The rest I'll leave to the parents," Harry was quoted by The Independent as saying.
The royal said he had already cuddled the Duke and Duchess of Cambridge's baby born on Monday and on Wednesday.
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The baby is destined to become George VII if his grandfather Prince Charles and father William choose to use their first names when they become monarchs, and not a middle name.
The infant prince's name is thought to be in tribute to the Queen's father, King George VI.
"When I saw him he was crying his eyes out like all babies do I suppose - it's fantastic to have another addition to the family," Harry said.
Asked what his nephew was like he held up his hands and said: "Well he's about that long and about that wide."
The Prince said it was too early to say whether Prince George takes after any member of the family, as: "He is about four days old so I think you can judge that one for yourself. I've no idea."
And when he was asked whether Prince William was up to the job of fatherhood, he replied: "Of course he is. Hopefully most people are.