A feeding trial was conducted to study the effect of dietary supplementation of potassium diformate (KDF) on pH of intestine, growth performance, feed utilization, hematological and serum biochemical parameters of Nile tilapia, Oreochromis niloticus (L.) fingerlings fed plant-based diets for 84 days. Three isonitrogenous (310 g kg^–1 crude protein) and isocaloric (18.70 MJ kg^–1 gross energy) diets were formulated. Each diet was supplemented with KDF at levels; 0 (control), 5 and 10 g kg^-1 diet.  The results showed that the pH value of diet was decreased by the addition of KDF. After 84-day, the pH of stomach and gut was significantly decreased by supplementation of KDF at level 10 g kg^-1 diet. The highest weight gain, specific growth rate, protein efficiency ratio and the best feed conversion ratio were recorded in fish fed either 5 or 10 g KDF kg^-1 diet. As well, addition of KDF significantly improved hemoglobin, hematocrit, red blood cells count, white blood cells, total protein, albumin and globulin compared with the control diet. Results of this trial indicated that, the addition of KDF as feed additive enhanced the growth performance, feed utilization, hematological and blood biochemistry parameters of Nile tilapia fed diet free of fishmeal.