This study examined the effect of a commercial probiotic (Lactobacillus acidophilus) on oxidative stress induced by different levels of stocking density in Nile tilapia fingerlings. Fish were divided into 6 groups: group1: control (90 fish/m³), group 2: control (90 fish/m³) with probiotic, group 3: high stocking density Ι (150 fish/m³), group 4: high stocking density Ι (150 fish/m³) with probiotic, group 5: high stocking density П (300 fish/m³), group 6: high stocking density П (300 fish/m³) with probiotic for 30 days. The results showed increase in the activities of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) by increased stocking density compared with their respective control. Activities of AST and ALT at high stocking density Ι, П with addition of probiotic decreased compared to their respective high stocking densities in liver and white muscles. The activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), glutathione reductase (GR) and glutathione S-transferase (GST) decreased by increasing stocking density with compared with their respective control in liver and white muscle.  However, they increased significantly in high stocking density Ι, П with probiotic compared with high stocking density Ι, П. The xanthine oxidase (XO) activity and malondialdehyde (MDA) level increased by increasing stocking density compared with their respective control and decreased in high stocking density Ι, П with addition of probiotic compared with high stocking density Ι, П. The data concluded that probiotics enhanced the oxidative stress caused by high stocking density in liver and white muscles in Oreochromis niloticus.