The movie features Stallone alongside a host of stars including Jet Li and Jason Statham, with cameos from action icons Bruce Willis and Arnold Schwarzenegger.
It debuted with earnings of $35 million (£23.3 million) at the weekend (ends15Aug10), and even Stallone is shocked The Expendables managed to beat Julia Roberts' feel-good film Eat Pray Love to the top of the U.S. box office chart, admitting his film was "up against some tough competition".
Now the veteran tough guy wants to build on the unexpected success by turning it into a franchise, and he's already working on a planned follow-up.
He tells the Hollywood Reporter, "It's plotted out in my mind's eye. I believe this group has to continue to evolve; it just can't become the same people. So how do you get new people introduced into the group, and how do you have some of the other people leaving? Those are the challenges."