Chemical analysis and nutritional assessment of defatted Garcinia mangostana seeds (DGMS) were undertaken to determine its suitability as an additive at 0.00g, 18.00g, 36.00g, 54.00g and 72.00g inclusion levels in diets and performance of Clarias gariepinus post juveniles. Proximate analysis of DGMS showed that the defatted seeds were high in carbohydrate (71.00 ± 0.79%) but low in protein content (8.10 ± 0.22%). The mineral element analysis detected different minerals with potassium as the highest (270.00ppm). All the fish increased in weight and length significantly (p<0.05) above the initial values though no significant differences were observed among treatments at 49days in all growth indices showing that the diets were similar in nutritional qualities and adequate for growth of fish. Hematological analysis showed increments in the final blood parameters at day 49 except MCH, MCV and heterophils and very high levels of leucocytes and platelets (p<0.05). No significant differences were observed in the haemoglobin content, AST, ALT, globulins and albumin in fish on DGMScontaining diets. Histology showed sub mucosal congestion in gill (36.00g DGMS), severe interstitial congestion in kidney (72.00g DGM) and vacuolations in liver in all groups except diet 4 (54.00g DGMS) and control (0.00g DGMS). However higher survival rates were observed in all DGMScontaining diets than the control