Dialkylated phthalates have been commonly used as plasticizers and a variety of applications. Phthalate diesters have been shown to be developmental and reproductive toxicants. It is very difficult to exactly estimate the dose of dialkylated phthalates taken up by the general population because of environmental contamination. Urinary metabolites of phthalates enabled to estimate internal exposure. The objective of this study was quantitative determination of phthalate metabolites by LC/MS/MS with on-line cleanup method to analyze phthalate metabolites in Korean children s urine. We employed LC/MS/MS with on-line enrichment and column-switching techniques for this biological monitoring. Metabolites determined were 4 primary metabolites; MEHP, MnBP, MiBP, MEP and 2 secondary metabolites of DEHP; 5-OH-MEHP), 5-oxo-MEHP. We analyzed children？s urine from 30 boys and 30 girls. The method detection limit of phthalate metabolites were 0.03 ng/mL for MEP, 1.05 ng/mL for MBP, 0.22 ng/mL for MEHP, 0.15 ng/mL for 5-OH-MEHP and 0.16 ng/mL for 5-oxo-MEHP, respectively. Switching Column LC/MS/MS was proven to be a useful tool to determine metabolites of phthalate diesters in human urine. The correlation among phthalate metabolites was very high and statistically significant, except MEP. The children？s age (months) was negatively correlated to the concentration of phthalate metabolites. The geometric mean concentration of phthalate metabolites (mg/g creatinine) in children？s urine were 25.5 for MEP, 130.3 for MnBP, 56.8 for MiBP, 19.5 for MEHP, 85.6 for 5-OH-MEHP and 83.1 for 5-oxo-MEHP, respectively. Levels of estimated daily intake of parent phthalate compounds (mg/kg bw/day) were 0.8 for DEP, 5.0 for DnBP, 1.9 for DiBP and 8.9~14.2 for DEHP, respectively. Estimated daily intake for DEP and DiBP were lower than those of other studies but the value for DEHP was higher than that of other study.