Jolie and Pitt have three children: 5-year-old Maddox, adopted from Cambodia; 23-month-old daughter, Zahara, adopted from Ethiopia; and 6-month old daughter, Shiloh, who was born to the couple in May.
"I want Mad (Maddox) to know that as our family grew and we all came together, we didn't just start having children, biological children," the 31-year-old actress said Thursday on ABC's "Good Morning America."
"Yes, we have Shiloh and it's been a wonderful experience, but we want to find another brother or sister in the world for our family."
She added: "I'm on the pill."
Jolie said it's important to balance how their children perceive their family.
"You know, now the questions are more when you have a mixed-race family, do you balance the races so there's another African person in the house for Z? So there's another Asian person in the house for Mad? Shiloh has Brad and I she can look at," the actress said.
"What's best for the children as they grow? ... We don't just want to have different children from different countries. That's not the point," Jolie said.
The couple and their three children were photographed for the upcoming issue of People magazine.
She said Pitt, who will be 43 on Monday, is a conscientious father and partner.
"He really enjoys them," Jolie said. "He woke up very, very early this morning and let me sleep in because I had this interview. ... Dealing with the two girls and bottles and food, which is not easy to do, on his own, for quite a few hours this morning so I could rest. ... He's just a really great partner, a great, great man."
Jolie co-stars in the upcoming Universal Pictures drama "The Good Shepherd" with Matt Damon and Robert De Niro.