Ronaldo, who won the European Golden Shoe last season after netting 40 times in La Liga, netted in the 24th, 71st and 87th minutes to complete yet another treble for Madrid - the £80million man scored at least three goals in a competitive match seven times last term.
Marcelo, Xabi Alonso and Kaka were also on the scoresheet for Jose Mourinho's men, who but for some fine saves from Zaragoza goalkeeper Roberto Jimenez and some indifferent finishing would have been celebrating an even bigger victory.
Mourinho said: "Real Madrid have to play well in order to win. This large difference on the score means we played a great game.
"But I can't ignore the reality of the match, which was that Aguirre experienced difficulties coming up with a team because he missed four important players: Braulio is very good, Ruben Micael has a quality the team doesn't, and I know Mateos and Juan Carlos very well because they were in our academy. Real Madrid played a serious and solid game."