Oil prices declined on concerns over the possible Greek exit from the Eurozone. 61.31% of Greeks voted "No" in the Sunday's referendum, while 38.69% of Greeks voted "Yes".
Greek Finance Minister Yanis Varoufakis has resigned after the referendum.
According to a Greek government official, Greek Prime Minister Alexis Tsipras spoke by phone on Monday with Germany's Chancellor Angela Merkel. He agreed to present new Greek proposal at Tuesday's European Union (EU) summit.
Traders expect the results of talks on the Iranian nuclear program. A deal could lead to a higher oil supply.
WTI crude oil for August delivery decreased to $53.91 a barrel on the New York Mercantile Exchange.
Brent crude oil for August fell to $58.28 a barrel on ICE Futures Europe.