The "Wall Street" star is undergoing an eight-week course of radiation and chemotherapy in New York.
Douglas said Tuesday during an appearance on David Letterman's "Late Show" that he had just finished his first week of radiation and chemotherapy.
"I'm treating this as a curable disease," he said. "It's a fight. I'll beat this."
The cancer remains above the neck and that means expectations are good, with an 80 percent or better chance of recovery, according to the 65-year-old.
Asked by Letterman about his personal habits, he said he had smoked and consumed alcohol, adding: "This particular type of cancer is caused by alcohol and drinking."
Catherine Zeta-Jones, Douglas' wife, told People the most difficult part of the ordeal has been seeing her husband fatigued "because Michael is never tired." She said she was furious doctors didn't detect the tumor earlier. The actor had spent months seeking a cause for throat and ear pain.
Douglas and Zeta-Jones have two children - Dylan, 10, and Carys, 7.
Douglas stars in the upcoming film "Wall Street: Money Never Sleeps," a sequel to "Wall Street" (1987), for which he won a best-actor Oscar.
The Associated Press contributed to this story.