There are various cerebral diseases that deform the cerebral shape with region specificity. So it is effective to quantify the deformation change of cerebral gyri. This study introduces new index called gyral deformation index (GDI) that is defined as a ratio of area of gyrus of interest to area of cerebrum in the defined projection plane. To calculate the gyral areas, this paper proposes a gyral labeling method in the projection plane using magnetic resonance images. The new method finds the boundaries between the gyri by optimizing deformable boundary models aided by fuzzy logic. The proposed method was applied to quantify the cerebral deformation of infants on a plane which is perpendicular to the longitudinal fissure. The comparison results with the manual delineation showed that the method labels gyri with a mean sensitivity of 92.8% and a mean false positive rate of 0.1% for 14 infantile subjects (3 weeks – 4 years 3 months old).