Cuoco, 24, and Galecki, 35, dated for two years before breaking up around the holidays last year, which happened to be six months before their characters broke up on the CBS series.
"It was a wonderful relationship but we never spoke a word about it and never went anywhere together," Cuoco tells CBS Watch! Magazine in the October issue.
The privacy factor, Cuoco says, may have been the cause of their eventual breakup. "That's not the kind of relationship I want — I don't want to be hiding. We couldn't do anything. It wasn't as fun as we wanted it to be."
However, Cuoco says their breakup did not affect their interaction on the set. "When you're dealing with two professionals who have been working since they were both 4 — and who kept their lives extremely private — we didn't want anything other than that."
"I'm so lucky it was a mutual ending and that we don't hate each other," she adds. "We've seen that happen on other shows."
The Big Bang Theory airs Thursdays at 8/7c on CBS.