Background: The deep cervical flexor (DCF) muscles have a crucial role in the management of neck pain. For preventing neck pain by activation of the DCF, craniocervical flexion (CCF) is an effective exercise. However, sternocleidomastoid (SCM) muscle is considered to affect negatively the activation of the DCF. SCM muscle which is an accessory muscle for respiration is activated differently depending on types of breathing patterns. It’s not certain that breathing patterns affect the SCM and DCF muscles thickness during CCF exercise. Objects: The purpose of this study was to investigate the influence of breathing patterns on the SCM and DCF muscles thickness during CCF exercise. Methods: Forty-five subjects participated in this study, and they were classified according to their breathing pattern, as follows: Costodiaphragmatic breathing (CDB) and upper costal breathing (UCB) groups. Ultrasonographic imaging of the SCM and DCF muscles was performed during five incremental levels of CCF during tidal breathing and expiration. Results: There was a significant interaction between the breathing pattern and the phase of CCF for percentage of SCM muscle thickness changes (p˂.05). In phase 1 CCF, a percentage of SCM muscle thickness changes was increased in the UCB group than in the CDB group (p˂.05). There was an increase in DCF muscles thickness with each additional CCF phase (p˂.05). Conclusion: Recruitment of SCM muscle was increased in the UCB group while performing CCF with a low intensity. There were no significant differences on DCF recruitment between the breathing pattern groups. Higher CCF exercise intensities elicited a higher DCF recruitment.