The countries will sign a new declaration to honour the 1957 treaty, which paved the way for the bloc's foundation.
British Prime Minister Theresa May is not attending the celebrations.
She plans to launch the Brexit process on Wednesday.
Supporters and opponents of the EU are planning marches in Rome.
Security has been tightened across the city following Wednesday's
Pope Francis welcomed the leaders to the Vatican on the eve of the summit.
In a speech, he said that all bodies risk dying if they do not look ahead, and thus urged the EU to approach the future with renewed vigour.
He also warned against "false forms of security" promised by populists who want to wall themselves off, and instead urged greater solidarity.
The original Treaty of Rome launched the European Economic Community, the EU's forerunner.
It was signed by six countries: Belgium, France, Italy, Luxembourg, the Netherlands and West Germany.
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