Objectives: The aim of this study was to use DKI to detect the microstructural change of the discs in matured normal rats. Methods: Total 24 normal SD rats (12 males/12 females) underwent DWI/DKI and T2 sequences with a 3T MRI scanner to get the values of ADC, FA, MD, Da, Dr, MK, Ka and Kr. The discs were categorized using a five-grade degeneration grading system in the 72-images. The height of the discs and the parameters in DWI/DKI were measured to compare between the different grades and sexes. The histological images and the images of fiber tracking were also done in the discs. Results: There were 30 Grade 1 and 18 Grade 2 in the discs. Compared with Grade 1, decreased ADC, increased FA and MM values were observed in Grade 2 (P < 0.05). By the ROC analysis of grades of the discs, there was low diagnostic accuracy in ADC value, while FA and MM showed higher accuracy. In Grade 1, there were lower ADC value, lower Dr, higher MK, Ma and Kr in male's group than them in female's group. There were no differences in the parameters except the ADC value in the two sexes in Grade 2. The different microstructure of the normal discs in the male and female rats had been proved by the histological images and the images of fiber tracking. Conclusion: DKI is a noninvasive and sustainable means to test the changes of intervertebral discs. The discs in Grade 2 were also found in the normal matured SD rat tails. The assessment of the grade of the discs in T2-images should be done before the experimental management. There was microstructural difference in the nucleus pulposus in the discs in Grade 1 and 2. FA and MK showed higher diagnostic accuracy. The laboratory rats should be the same sex because the microstructure of the normal discs weren't the same. (C) 2017 Elsevier Inc. All rights reserved.