It happened near the Greeley Family Funplex around 4 p.m.
Officials say Trooper Dan Haley was with his family at the park near the Funplex, when he noticed some boys playing on thin ice.
"Mentioned to my wife one time, just waiting for something to happen because it might just happen, you have to be ready for it," Haley said.
Brenndan Daviet, 10, wasn't supposed to be playing anywhere near the pond, his mother Bobbi told 9NEWS.
Daviet was playing with a few older boys.
"Somehow my friend's phone came out of his pocket, somehow it slid across the lake," Daviet told 9NEWS. "I tried to get it. That's when the ice broke. I screamed 'Help.'"
Trooper Haley said he ran towards the water and then crawled on ice to reach the boy. The ice broke under him and they both ended up in the freezing water.
"The whole time crawling out, I was telling him, 'Look at me.' He was staying amazingly calm for what was going on at his age," Haley said.
Haley said the boy was still floating and doing a good job at it. He broke the ice for both of them to swim to shore until he turned around and he saw Daviet drowning. Haley pulled him up by his coat.
"I don't know how I did it. I was able to grab his leg, his waist, whatever, and throw him up on to the ice," Haley said.
Daviet was able to crawl to shore. Haley made it safely as well.
"It is dangerous; it's part of the job," Haley said. "I held my right hand and I gave my oath to protect the citizens of Colorado, whether I'm on duty or off duty. Also, it's a kid out there. I got a daughter. I'd want the same for my daughter if someone saw that."
"I'm just so thankful for him," said Bobbi Daviet, Brenndan's mother. "He's our angel definitely. I don't know what we would've done if he wasn't here. Probably wouldn't have had Brenndan."
Daviet said Brenndan was grounded. But the terms of his punishment were not yet clear, she said. The family is still trying to get over almost losing him.
Both were transported to the Northern Colorado Medical Center where they were treated and released for hypothermia.
Trooper Haley has been with the Colorado State Patrol for three years and is currently assigned to the Homeland Security Section.
(KUSA-TV © 2010 Multimedia Holdings Corporation)