The Italian coach says the Portuguese is a joy to work with and singled him out as one of the finest he has seen
The new Blancos coach, who arrived at the Santiago Bernabeu from Paris Saint-Germain earlier this summer, was speaking ahead of his side's Guinness International Champions Cup campaign.
And when quizzed on the credentials of the Portuguese, Ancelotti was more than glowing in his appraisal of the 28-year-old.
"I've trained tons of talented players," Ancelotti said. "But Ronaldo is a phenomenon. He has an incredible talent, physical quality, and I'm extremely happy to be able to coach him.
"He's very professional, and I'm convinced he's going to have a great season."
Ronaldo finished second in the Liga scoring charts last season, netting 34 goals in 34 matches. Only Ballon d'Or winner Lionel Messi scored more, tallying 46 times.
Real Madrid kick off their involvement in the ICC with a match against the LA Galaxy at University of Phoenix Stadium in Arizona on Aug 1 and face Paris Saint-Germain in a friendly in Sweden on Sunday.