The aim of this study was to investigate the optimal age for starting cranial-remolding-orthosis therapy in children with deformational plagiocephaly. Medical records of 310 patients with deformational plagiocephaly were retrospectively reviewed and the initial and final cranial vault asymmetry index (CVAI), age when starting therapy, duration of therapy, mean change of CVAI, improvement rate, and treatment success were analyzed. We compared outcomes according to the groups divided by ages starting therapy. There were no significant differences in improvement rate and duration of cranial-remolding-orthosis therapy among patients starting therapy at the age of 3, 4, and 5months. However, when starting therapy after the age of 6months, the rates of CVAI improvement were significantly lower and the duration of therapy was significantly increased. Considering the spontaneous resolution effect according to the head growth nature, the age 5month is the optimal period to start cranial-remolding-orthosis therapy for deformational plagiocephaly.