The victim, who has not been identified, was stabbed once in the chest. He was taken to Sutter Roseville Medical Center, where he was pronounced dead.
Police have arrested Zachary Raymond Kachmar, 20, of Rocklin. He was taken to Placer County Jail, where he is being held without bail.
At 10:05 p.m. Saturday, police were called to a stabbing at a home in the 6200 block of Swan Court, where a party was being held in the backyard. Officers found the victim inside the house, bleeding from a chest wound.
Investigators later determined that Kachmar and the victim had apparently argued at the party, and Kachmar allegedly pulled out a pocket knife and stabbed the victim once.
Kachmar knew the victim's son, according to Rocklin police Capt. Lon Milka.
It was not known Sunday what the argument was about, but police have recovered the knife.
The death marks the first homicide in Rocklin since 2007.
"Rocklin has always been a safe city," said Milka. "This is not a reflection on the good people living here. It is not a random act."
PHOTO: Rocklin police arrested Zachary Raymond Kachmar, 20, of Rocklin in the stabbing death of a 51-year-old man.