The jury will recommend either a death sentence or life without parole for the Modesto fertilizer salesman who was convicted of killing his pregnant wife, Laci, and their unborn child in 2002.
The formal sentencing was slated for late February.
Stanislaus County Superior Court Judge Alfred Delucci told jurors in his instructions they could consider any extenuating circumstances and any sympathy they might have for Peterson and his family in their deliberations.
Peterson attorney Mark Geragos asked the jury to spare his client's life and not commit an act they could not take back.
Prosecutors earlier Thursday called Peterson a "monster" and urged a death sentence.