The countries will sign a new declaration to honour the 1957 treaty, which paved the way for the formation of the EU.
But British Prime Minister Theresa May is not attending as she plans to launch the Brexit process on Wednesday.
Security has been tightened across the city following Wednesday's terrorist attack in London.
Pope Francis welcomed the leaders to the Vatican on the eve of the summit with a reception.
The original Treaty of Rome launched the European Economic Community, the EU's forerunner.
It was signed by Belgium, France, Italy, Luxembourg, the Netherlands and the then West Germany.
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