A New Jersey man killed his father and his grandmother — and then threw a party, prosecutors said yesterday.

Dwayne Flourney, 26, stabbed his grandmother, a retired schoolteacher, at the family’s home on a tree-lined, suburban Maplewood street Saturday afternoon, the Essex County Prosecutor’s Office said.

When his dad arrived home later in the afternoon, Flourney fatally knifed him, too, authorities said.

He then allegedly hid the bodies, stashing grandmother Sandra Flourney, 76, in the trunk of a car parked in the home’s garage and leaving his father, Brian Flourney, 52, in a chair on the sun porch, covered by a sheet.

Later, he invited pals over to party.

The friends apparently didn’t realize the corpses were in the house, authorities said. After sleeping there, Flourney drove off Sunday in his father’s car.

Brian Flourney’s colleagues in the Union County Public Defender’s Office became worried when he failed to show up Monday for his job as an investigator.

They sent cops to his home, where they discovered the bodies.

A knife was recovered at the house, prosecutors said.

Dwayne Flourney was arrested Tuesday night at a female friend’s house in Phillipsburg, NJ.

He is being held in the Essex County Jail in lieu of $1 million bail.

He is charged with two counts of murder, as well as weapons offenses.

The slayings stunned Flourney’s neighbors.

“This is just shocking,” Luchner Maxi, 18, told the Newark Star-Ledger.

“This is a great street. Nothing really happens on this street.”