Recently, jaw size was changed from 4 mm aper- ture (J4) to 1 mm (J1) in TomoTherapy Hi-Art II® (TomoThe- rapy Inc., Madison, WI) to improve the longitudinal (IEC-Y) resolution of megavoltage computed tomography (MVCT) images. This study attempted to conduct a comparative analy- sis on the effect of jaw aperture size of MVCT on image quali- ty and dose. Also, this study investigated the clinical effect of jaw aperture size on highly radiosensitive lens dose. MVCT image quality test (image noise, uniformity, contrast linearity, and spatial resolution) and multiple scan average dose (MSAD) were measured to examine the effect of change in jaw aperture size. A head phantom and photoluminescence glass dosimeters (PLDs) were used to measure exposed lens dose (cGy). For image quality, the change in jaw aperture size did not have a significant effect on image noise, uniformity, con- trast linearity, and spatial resolution although showing slight improvement of image quality. FHWM was 6.7 mm and 4.1 mm in J4 and J1, respectively, which represents enhancement in the longitudinal resolution of MVCT image. MSAD of cen- ter point was approximately 0.69-2.32 cGy (peripheral: 0.83- 2.48 cGy) in J4, and 0.85-2.81 cGy (peripheral: 1.05-2.86 cGy) in J1. The measured lens dose using PLDs ranged 0.92-3.36 cGy and 1.06-3.91 cGy in J4 and J1, respectively, indicating dose increase for the narrower jaw aperture size. An addition- al dose of 3.86 cGy, 1.92 cGy, and 1.22 cGy was delivered for a pitch of 1, 2, and 3, respectively during MVCT scan per frac- tion in head and neck treatment plans. Therefore, 15% in- crease in imaging dose with 1 mm aperture jaw should be allowed for compared to that with previous 4 mm aperture jaw.