The study investigated the effect of Vernonia amygdalina (VA) leaf supplementation on organ weight and gut microbial count of Thryonomys swinderianus. Thirty-six weaner grasscutters of mixed sexes were allotted randomly to six treatments (A, B, C, D, E, and F) in a completely randomized design for a nutritional trial of 8 weeks. At the end of the experiment, caecum, kidney, liver, lungs, intestine and the heart were excised to determine their weights and the intestinal/caecal contents and scrapings were obtained to determine the fungi and bacteria population. Data collected were analyzed using one-way Analysis of Variance (ANOVA). The VA supplementation had significant effect (p<0.05) on relative organs with a range of 0.47 – 0.60%, 0.40 – 0.62%, 1.20 – 2.83%, 0.52 - 0.58%, 5.53 – 12.91%, 714.00 – 960.00g, 494.00 – 781.00g and 479.00 – 747.00g for lungs, heart, liver, kidney, intestine, live weight, eviscerated and carcass weight of the grasscutter respectively, with a smaller relative weight of the kidney from 3g/kg VA supplementation upwards. The serum biochemical parameters were also significantly affected by VA supplementation. There was a decrease in alkaline phosphatase, alanine aminotransferase and aspartate aminotransferase but were within the normal range required for rodents. Total protein and Albumin were significantly improved by the supplement while total bilirubin was variable with a significant decrease prior to 4gVA supplementation. Microbial populations in the intestine and ceacum were reduced to varying extent by the supplement. It could be concluded that VA supplementation reduced intestinal and caecal microbes and increased the live and carcass weights but decreased relative weights of selected organs in the grasscutter.