Microbial susceptibility of the denim fabric sized by starch raises some skin health concerns about this extensively used fabric. In this work, biocompatible antibacterial denim fabric is prepared by optimized green synthesis of the copper(I) oxide (Cu 2 O) nanoparticles on the cotton fibers. The copper oxide nanoparticles were synthesized without using any toxic chemical to reduce the copper salt. As an alternative, the waste of the sugar refinery factory, molasses, was used as eco-friendly and cost-efficient material. The antibacterial activity of the samples against Candida albicans as model fungus, Escherichia coli as Gram-negative, and Staphylococcus aureus as Gram-positive model bacteria is investigated besides cytotoxicity; also, the synthesis process is optimized regarding the most biocompatible and antibacterial nanocomposite. The characterizations prove the effectiveness of the molasses for reduction of the copper salt and formation of the copper(I) oxide (Cu 2 O) nanoparticles. Synthesis of the copper(I) oxide (Cu 2 O) nanoparticles has a positive effect on the mechanical properties of the nanocomposite. The results of this work introduce a green method for sustainable development of the fabric industry; it also presents a potential application for molasses and reutilizing it.