Biomonitoring of nonylphenol (NP), an endocrine disrupting chemical, is required in Korea to perform its proper regulation. Thus, we analyzed exposure levels of nonlyphenol (NP) in breast milk from the mothers who delivered babies within 10 days (N=325). We analyzed free and total forms of NP in breast milk with LC/MS/MS (LOD, 0.5 ppb). In addition, we obtained questionnaires concerning lifestyle from the subject. As results, ranges of total NP were <LOD-23.4${mu}g/L$ (median, <LOD) and the estimated exposure level of the NP was lower than TDI (tolerable daily intake)s. However, we found that the levels of total NP in the mothers, who had some diseases such as thyroid disorders, were higher than those in healthy mothers (sick mothers, N=34, $3.51{pm}4.98{mu}g/L$ vs. normal, N=281, $2.07{pm}3.76{mu}g/L$; p<0.05). In conclusion, we suggest that exposure monitoring of NP should be continuously performed, even though the risks of NP are not clear, yet.