Brad Pitt and Angelina Jolie, who have been together for seven years and have six children, are said to be setting their sights on a spring wedding.
The Hollywood A-list couple are discussing holding the service in Pitt's hometown of Springfield, Missouri.
In addition, the duo has set their hearts on a spiritual ceremony incorporating traditions from the different countries where their children were born.
Three of the couple's brood are adopted - Maddox, ten, is from Cambodia; Pax, eight, is from Vietnam and Zahara, seven, is from Ethiopia.
Their biological daughter Shiloh, five, was born in Namibia and three-year-old twins Vivienne and Knox in Nice.
"The ceremony will be low-key with just family and friends invited," the Daily Mail quoted a friend of the couple as saying.
"Angelina wants her father Jon Voight and her brother James Haven to be a part of the day.
"They are keeping their cards close to their chest. The ceremony will by no means be a showbiz event. They want total privacy.
"All the children will be involved - Brad and Angelina have been researching weddings in the countries where their children were born," the friend said.
The source also said that since neither of them is religious, the ceremony will be spiritual.
"It won't be a church service but it will incorporate all the cultures within their family," the source said.
The 48-year-old actor raised the prospect of marriage last week, telling a Hollywood magazine that they'd "like to and it seems to mean more and more to our kids."
The couple, nicknamed Brangelina, started dating after meeting on the set of 'Mr and Mrs Smith' in 2004.
Pitt was previously married to 'Friends' star Jennifer Aniston, while Jolie has been married to actors Jonny Lee Miller and Billy Bob Thornton.