Previous studies suggested that myelinated axons innervating rat molar pulps undergo morphological changes in their peripheral course. However, little information is available on the morphological feature of the parent axons at the site of origin. We therefore investigated the size of the myelinated parent axons and their morphological features at the proximal sensory root of the trigeminal ganglion by horseradish peroxidase (HRP) injection into rat upper molar pulps and subsequent light and electron microscopy. A total of 248 HRP-labeled myelinated axons investigated were highly variable in the size. Fiber area, fiber diameter, axon area (axoplasm area), axon diameter (axoplasm diameter), and myelin thickness were 11.32 ± 8.36 μm2 (0.80~53.17 μm2), 3.99 ± 1.53 μm (1.08~9.26 μm), 8.70 ± 6.30 μm2 (0.70~41.83 μm2), 3.13 ± 1.13 μm (0.94~7.20 μm) and 0.43 ± 0.23 μm (0.07~1.06 μm), respectively. The g-ratio (axon diameter / fiber diameter) of the labeled axons was 0.79 ± 0.05 (0.61~0.91). Axon diameter was highly correlated with myelin thickness (correlation coefficients, r=0.83) but little correlated with g-ratio (r=−0.33) of individual myelinated parent axons. These results indicate that myelin thickness of the myelinated parent axons innervating rat molar pulps increase with increasing axon diameter, thus maintaining a constant g-ratio.