In this study, the antimicrobial effects of an onion peel extract prepared using subcritical water extraction (SWE) were assessed for possible development into new bio-functional materials. The extraction temperatures were controlled to 110 and $160^{circ}C$. At 0.15, 0.3, 0.6, and 1.2 mg extract/mL of broth, the growth inhibition and bactericidal activity of SWE extracts against Bacillus cereus KCCM 40935 and KCCM 11341 were compared with those of ethanol and hot-water extracts. In the case of B. cereus KCCM 40935, it appeared that over 0.6 mg/mL of SWE ($110^{circ}C$) extract exerted a bactericidal effect, and 1.2 mg/mL of SWE ($160^{circ}C$) extract exerted a bacteriostatic effect during culturing, and also that B. cereus KCCM 11341 was more resistant than B. cereus KCCM 40935. Furthermore, our results demonstrated that the death time of $10^7CFU/mL$ of B. cereus KCCM 40935 treated with SWE ($110^{circ}C$) extract at 1.2 mg/mL was 60 min at maximum in 0.8% NaCl. Additionally, the cells damaged by SWE extract were observed with a SEM. It was suggested that an extract of onion peels prepared via SWE ($110^{circ}C$) could be used as a functional biomaterial for the food or pharmaceutical industries.