The impact of the liberation of copper bearing particles on copper recovery in the flotation has been the subject of research in this paper. Tests have shown that grinding of material highly impacts the recovery rate in the flotation process. Results of flotation of smelter slag samples with different contents of grain size fraction -74 mu m: 50%, 60%, 70%, 80%, and 90%, have shown that by an increasing the content of the -74 mu m fraction in the feed, the recovery of copper was increasing as well. The highest recovery rate of copper was obtained with 90% of grain size fraction -74 mu m in the feed. The microscopic analysis of concentrates have shown that increased content of grain size fraction -74 mu m, was followed by increased contribution of liberated particles in the 0 to 70 mu m fraction, while the contribution of middlings had been decreasing.