Cu2+, Zn2+, Fe2+ and I-are often supplemented to the diet of suckling and early weaning piglets, but little information is available regarding the effects of different CU2+, Zn2+, Fe2+ and I-mixtures on bacteria growth, diversity and fermentation characteristics of fermented liquid diet for piglets.Pyrosequencing was performed to investigate the effect of CU2+, Zn2+, Fe2* and I-mixtures on the diversity, growth and fermentation characteristics of bacteria in the liquid diet fermented with Bacillus subtilis and Enterococcus faecalis under air-tight condition.Results showed that the mixtures of Cu2+, Zn2+, Fe2+ and I-at different concentrations promoted Bacillus growth, increased bacterial diversity and lactic acid production and lowered pH to about 5.The importance of Cu2+, Zn2+, Fe2+ and I-is different for Bacillus growth with the order Zn2+> Fe2+>Cu2+> I-in a 21-d fermentation and Cu2+>l->Fe2+>Zn2+ in a 42-d fermentation.CU2+, Zn2+, Fe2+ and I-is recommended at a level of 150, 60, 150 and 0.6 mg/kg respectively for the production of fermented liquid diet with Bacillus subtilis.The findings improve our understanding of the influence of trace elements on liquid diet fermentation with probiotics and support the proper use of trace elements in the production of fermented liquid diet for piglets.