As an attempt for reducing the harmful impacts of throwing the used-engine oil (UEO) in the environment on marine, human and underwater lives and agriculture production, this study have been undertaken, aiming to adopt such waste material in concrete industry as a chemical admixture. Therefore, an experimental program was designated initially to investigate the effectiveness of UEO on the various fresh properties of concrete (consistency, rate of slump loss, setting time and air content). The performance of concrete containing UEO in its hardened state was then thoroughly studied, through assessing the various mechanical and microstructure-related properties, namely, 28 day-compressive strength, surface hardness, homogeneity, sorptivity, porosity and degree of hydration. Ten OPC concrete mixes and another ten OPC paste mixes made with different contents of UEO, water reducing admixture and air-entraining admixture (0, 0.15, 0.30 and 0.60%, by cement mass) were consequently prepared. The results showed that utilizing of UEO in OPC mixes has reasonably altered its fresh parameters, where, an increase in initial slump and air content, and a decrease in rate of slump loss and initial setting time were found. A slight reductions in the 28-day compressive, homogeneity, density and degree of hydration, and an increase in porosity and rate of water inflow into cover zone were noted when UEO was induced into OPC mixes. The results also indicated that the fresh and hardened characteristics of OPC concretes containing either UEO or air entraining admixture are comparable.