This study investigates the actions of the cricothyroid joint for F0 changes based on high-resolution MRI and 3D image analysis. The data from a male speaker's phonation at two fundamental frequencies were analyzed with a 3D pattern matching method to obtain displacement and angular changes of the thyroid and cricoid cartilages. Results show displacement of the two cartilages relative to each other in 3D. The largest difference between 110 Hz and 165 Hz was found to be 1.2 mm horizontally and 0.6 mm vertically with respect to the translation of the cricothyoroid joint. This action is interpreted to be caused by the contraction of the cricothyroid muscle which draws the thyroid and cricoid cartilages together, and results in stretching of the vocal folds.