Ronaldo's three goals, and one from Sami Khedira, gave Real Madrid a 4-0 win on Wednesday night when they played the last 20 minutes with just 10 men in the last-16 stage of the tournament, reports Xinhua.
Real Madrid coach Jose Mourinho decided to start with Iker Casillas in goal ahead of Antonio Adan, who had been first choice in the last two matches.
Casillas earned a round of applause from the fans in the Santiago Bernabeu Stadium who, nevertheless, booed and whistled Mourinho when his name was given on the stadium PA system.
Two goals from Ronaldo were enough to see Madrid into the last eight. His first after just two minutes was a spectacular effort from 25 yards, which dipped and curled away from Celta keeper, Sergio Alvarez.
The visitors paid the price for leaving several key players, such as striker Iago Aspes, out of their starting 11 when Ronaldo doubled Madrid's lead after 22 minutes and it was not until the second half, with Aspes on the pitch that Celta began to threaten the Madrid goal.
Sergio Ramos was sent off for two yellow cards with 20 minutes remaining and Celta began to pressure the Madrid goal, but were denied by several good saves from Casillas.
Ronaldo completed his hat-trick after 87 minutes and Khedira rounded off the scoring a minute later and Real Madrid will now face Valencia for a place in the semi finals.
Meanwhile, Zaragoza also qualified for the last eight thanks to a 3-0 aggregate win over Levante. Zaragoza kicked off on the back of a away win in the first leg and had few problems in sealing their qualification.
Franco Zuculini opened the scoring after 23 minutes and Fernando Montanes doubled their lead on the hour to assure their place in the next round.
Wednesday's other game saw Mallorca win 2-1 in Sevilla, a result that had little importance given that Sevilla had won 5-0 in Mallorca in the first leg.
Manu del Moral had put Sevilla in the lead after 21 minutes, but Brandon Thomas and Alfaro turned the result around to give Mallorca a win that will at least serve to raise their morale after a win and a draw in their last two league matches.
Tuesday saw Malaga and Valencia qualify for the last eight of the competition. Malaga will almost certainly face Barca, who defend a 2-0 first leg lead against Cordoba in the Camp Nou Stadium.
