Purpose: The pathophysiology of congenital muscular torticollis (CMT) is that the sternoclavicularmastoid (SCM) is shortened on the involved side by fibrosis, leading to an ipsilateral tilt and contralateral rotation of the face and chin. The aim of this study was to examine the effect of physical therapy and develop a mass diameter prediction model for infants with CMT. Methods: Fifty six patients were diagnosed with CMT between April 2003 and December 2008. Infants with neurological complications, and spasmodic and ocular torticollis were excluded. Physical therapy was applied to those masses in the SCM muscles of those infants after checking their physical findings and the diameter of the mass with ultrasonography. Their physical findings and mass diameter was reevaluated when their neck tilt was under $5^{circ}$. Results: The mean age when physical therapy was started was 35 days. After a mean 90 days of treatment, the subjects showed improvement in the neck tilt. Subjects whose neck tilted above $15^{circ}$ showed significant improvement in neck tilt decreased their mass diameter (p<0.01). Facial symmetric infants showed a shorter recovery duration than the facial asymmetric infants (p<0.05). A mass decreasing model based on the diameter of the mass, facial symmetry or not and the physical therapy start day after birth was developed by linear regression. Conclusion: Physical therapy is an effective treatment for CMT. The change in the diameter of the mass on the SCM muscles after treatment can be predicted.