The same jury found Stepp guilty last week of first-degree murder and sexual assault in the death of his 10-month-old stepdaughter Cheyenne Yarley and was trying to reach a decision on life in prison or the death penalty.
Because it couldn't reach a sentencing recommendation, Superior Court Judge Osmond Smith said under the law he would automatically impose the lesser sentence of life in prison without the possibility of parole.
The case will automatically be appealed.
Cheyenne Yarley was assaulted and killed Nov. 8, 2009. Jurors heard graphic testimony during the trial as experts described the baby's injuries.
Attorneys for Stepp, an Iraq war veteran, said he suffers from posttraumatic stress syndrome related to his combat experiences and was self-medicating with drugs and alcohol the night he killed the child.