Hagman, who played central character J.R. Ewing, a scheming oil baron and cattle rancher, said he is looking forward to the event.
"I may not be able to do the 40th," said Hagman, 76, in an interview with The Dallas Morning News. "Thirty years is pretty good, and the show is still very popular."
The reunion at the ranch in suburban Parker, north of Dallas, is open to the public. Tickets go on sale Aug. 22 and will cost between $100 and $1,000. The event will include fireworks, a country music concert, a question-and-answer session with the cast and tours of the mansion.
The show is perhaps best known for the "who shot J.R.?" cliffhanger that concluded the 1979-1980 season in which Hagman's character was blasted within an inch of his life. (The shooter turned out to be J.R. Ewing's sister-in-law, Kristin.)
"Dallas" featured the antics of a wealthy Texas clan that punched, slapped, drank, cavorted and schemed to gain control of the family fortune. The reunion promises to be tamer.
"Larry and I will have a bowl of Mueslix and a small shot of prune juice and be in bed by 9 o'clock," Duffy, who played Bobby Ewing, told the newspaper. "We'll let people file by the bed and watch us go to sleep. I think we can hang with the best of them until midnight."