WordNet (r) 3.0 (2006):

peace of mind
    n 1: the absence of mental stress or anxiety [syn: peace,
         peacefulness, peace of mind, repose, serenity,
         heartsease, ataraxis]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "peace of mind":
   Buddha-like composure, Oriental calm, acceptance, calm disposition,
   calm of mind, calmness, comfort, composure, content, contentedness,
   contentment, cool, coolheadedness, coolness, countenance, ease,
   easy mind, entire satisfaction, euphoria, fulfillment, happiness,
   icy calm, imperturbation, indisturbance, mental composure,
   peace of God, peace of heart, peacefulness, philosophic composure,
   philosophicalness, philosophy, placidity, quiet, quiet mind,
   quietude, reconcilement, reconciliation, resignation, sangfroid,
   satisfaction, serenity, soothingness, tranquillity, unruffledness,
   well-being