The present study aims to determine the contents of total phenolic compounds and antioxidant activity in three food processing by-products extracts including eggplant peel extract (EPP), onion skin extract (OSE), pomegranate peel extract (PPE) and their mixture. Also, exploring the relationship(s) between the content of these extracts of phenolic compounds and their antioxidant  activities, was in the scope of this investigation.The selected phyto-extracts showed considerable differences in antioxidant activity (AA) and the OSE recorded the highest value (90.61%) followed by ME (89.69), PPE (86.03) and EPE (81.04). Also, the total phenolics content was ranged 98.95 to 298.56 mg GAE/ g. The OSE was exhibited the highest content of total phenolics while EPP recorded the lowest value. Furthermore, when all selected phyto- extracts were included in the statistical analysis, there was a positive and highly significant (p≤ 0.05) relationship between total phenolics and antioxidant activity. The highest value was recorded for OSE (r²= 0.8982, p≤ 0.05) followed by PPE (r²=0.8633, p≤ 0.01), EPE (r²= 0.8471, p≤ 0.05) and ME (r²= 0.7840, p≤ 0.05), respectively. The samples also recorded other biological activity i.e. inhibition of low density lipoprotein (LDL) oxidation. In conclusion, we recommended to pay attention in the future to carry out more research in the area of therapeutic nutrition with food processing by-products with the high content of different categories of bioactive compounds and extended their applications in human diets, industrial and medical applications instead of the synthetic antioxidants/chemicals used which have induced healthy hazards and side effects for the human being.