Background and Objectives: Biofilm formed by Proteus mirabilis strains is one of the most important medical problems especially in the case of device-related urinary tract infections. This study was conducted to evaluate the bacteriocin produced by a marine isolate of Bacillus sp. Sh10, for it's in vitro inhibitory activity against pre-formed biofilm and in interference with the biofilm-forming of two biofilm-producing bacteria ( P. mirabilis UCa4 and P. mirabilis UCe1). Materials and Methods: Sensitivity of two biofilm-producing bacteria ( P. mirabilis UCa4 and P. mirabilis UCe1) to bacteriocin, was investigated in planktonic and biofilm states by cell viability and crystal violet assay, respectively. Scanning electron microscopy (SEM) was also performed to determine the effect of bacteriocin on the morphology of the cells associated with biofilm. Results: It was found that bacteriocin possessed bactericidal activity to biofilm-forming isolates in the planktonic state. However, bacteriocin interferes with the formation of biofilms and disrupts established biofilms. Bacteriocin reduced biofilm formation in the isolates of P. mirabilis UCa4 and P. mirabilis UCe1 with SMIC50 of 32 and 128 μg/mL, desirable SMIC50 of bacteriocin for biofilm disruption were 128 and 256 μg/mL, respectively. The SEM results indicated that bacteriocin affected the cell morphology of biofilm-associated cells. Conclusion: The present findings indicated that bacteriocin from Bacillus sp. Sh10 has bactericidal properties against biofilm-forming isolates of P. mirabilis UCa4 and P. mirabilis UCe1 and has the ability to inhibit the formation of biofilm and disrupt established biofilm.