Miller, who came to the Marlins as part of the trade that sent Dontrelle Willis to Detroit, is competing with Mark Hendrickson, Rick VandenHurk and Ricky Nolasco to start Florida's opener March 31 against the Mets.
He was 5-5 with a 5.63 ERA in 13 starts last season for the Tigers.
"That was a very good outing for him," Marlins manager Fredi Gonzalez said of the sixth overall pick in the 2006 draft. "He is getting better every time out. He's making adjustments."
Miller was encouraged to have a solid outing with many of Houston's top hitters, including Carlos Lee and Lance Berkman, in the lineup.
"They had the lineup for the most part you've got to agree that they're going to go with," he said. "I think that's one of the good things of a game like this is that they're playing all their big guys."
Miller, who is from Gainesville, Fla., had a personal cheering section of more than a dozen friends and relatives on Friday.
"There's so many people here it's unbelievable," he said. "My family is pretty much all in the central and south Florida area. I left a lot of tickets."
Astros ace Roy Oswalt allowed his most hits of the spring with 10. He allowed four runs and walked two with three strikeouts in five innings.
"I don't think it's good," Oswalt said of his performance. "But we wanted to throw 100 pitches and though I got through it quicker than I wanted to, we got 100 pitches. Spring training is to build up arm strength and that's what we're trying to do."
He was unhappy with the walks. It was the first time this spring he had walked more than one batter in a game.
"My big concern was walking the two guys," he said. "That peeved me more than anything. Other than that, not too bad."
Marlins second baseman Dan Uggla had a double, a home run and four RBIs on Friday to double his RBI total for the spring. Mike Jacobs had a double and added a homer in the ninth inning for Florida.
Houston's Jose Cruz Jr. continues to make a strong case for making the team and had a double and a home run on Friday.
Notes: Florida's Cody Ross injured his knee on Friday but Gonzalez said it was not serious and he should be fine after a day of rest. ... Astros catcher J.R. Towles, who has been out since injuring a hamstring March 8, played six innings in a minor league game on Friday and is scheduled to return to the Astros starting lineup Saturday. ... LHP Wandy Rodriguez, who strained a left oblique muscle a week ago, pitched one inning in a minor league game on Friday and should be OK.