Speaking with US TV host Graham Bensinger, the prince told how 'deeply affected' his father, the late Prince Rainer, was after the tragedy, when then 52-year-old Hollywood actress crashed her car.
'He wasn't quite the same man as he was before the accident,' Albert admitted.
The 59-year-old - who shares twins Jacques and Gabriella with wife Princess Charlene - also detailed his disbelief at the time that his mother was really gone.
'Of course you think things are going to improve and it's not as bad an accident as you thought it was,' he said.
For the full story see this weeks issue of New Idea.