OBJECTIVE To study the thickness and width of extraocular muscles by high-resolution magnetic resonance imaging (MRI). METHODS Cross-sectional study. Sixty eight normal subjects between 2.0 and 50.0 years of age, mean age was (21.2 ± 2.1) (SD) years old, male 34, female 34. Extraocular muscles in the orbits were scanned with SE/T2WI coronal and oblique-coronal planes perpendicular to optic nerve, and within 1.5 mm thick planes. The thickness and width of the extraocular muscles in the maximum planes were measured in 68 normal subjects on oblique-coronal planes MR images. The vertical diameter of the medial rectus muscle and lateral rectus muscle indicated width, horizontal diameter indicated thickness. The vertical diameter of the superior rectus muscle and inferior rectus muscle indicated thickness, horizontal diameter indicated width. Vertical diameter of the muscle belly is from the top edge to the bottom edge, horizontal diameter of the muscle belly is from the innermost to the outermost edge. The differences in diameters of bilateral extraocular muscles were evaluated by the independent sample t statistical test. Superior oblique muscle was considered as a reference in each subject to calculate the ratio of the other rectus muscles and the superior oblique muscle in diameter and to compare the relationship of the extraocular muscles in the same plane. RESULTS The extraocular muscles of normal subjects were symmetrical on the two sides. There were no statistically significant differences between the bilateral extraocular muscles (medial rectus muscle, lateral rectus muscle, inferior rectus muscle and superior rectus muscle) in the thickness and width (t values respectively were 0.043, 0.025, 0.043, 0.032, 0.001, 0.601, 0.667, 0.021, 0.032, 0.005, all P > 0.05). The ratio of the rectus muscles (medial rectus muscle, lateral rectus muscle, superior rectus muscle, inferior rectus muscle) and the superior oblique muscle in thickness respectively were (1.15 ± 0.14), (1.04 ± 0.14), (1.07 ± 0.19), (1.39 ± 0.22). The relationship of the thickness of extraocular muscles in the maximum planes were: inferior rectus muscle > medial rectus muscle > superior rectus muscle > lateral rectus muscle, and the thickness of 91% superior rectus muscle was close to lateral rectus muscle. The ratio of the rectus muscles (medial rectus muscle, lateral rectus muscle, superior rectus muscle, inferior rectus muscle) and the superior oblique muscle in width respectively were (1.71 ± 0.27), (2.07 ± 0.28), (1.62 ± 0.29), (1.50 ± 0.21). The relationship of the width of extraocular muscles in the maximum planes were: lateral rectus muscle > medial rectus muscle > superior rectus muscle > inferior rectus muscle. CONCLUSIONS The thickness and width of all the extraocular muscles follows a certain rule. Measurements of extraocular muscles using MRI should be done on coronal and oblique-coronal planes MRI scan simultaneously. The sizes of extraocular muscles can be assessed by observing the symmetry of the extraocular muscles on the two sides on coronal plane and the usual rule of extraocular muscles on oblique-coronal plane.