Abstract: A simple and effective method for the determination of trace amounts of methylmercury (MeHg+) and inorganic mercury (Hg2+) in seafood was developed by online microcolumn separation/preconcentration combined with inductively coupled plasma optical emission spectrometry (ICP-OES). It was found that Hg2+ could be quantitatively adsorbed by YPA(4) resin from pH 7.0 to strong acidic medium (6 mol L-1 HCl) and that MeHg+ was retained by the YPA(4) microcolumn only at pH 1.0-7.0. Therefore, a strong acidic medium (about 5 mol L-1 HCl), which could liberate mercury species from biological samples, was used to directly separate inorganic Hg2+ from total Hg, and MeHg+ in effluent was retained by YPA(4) column after the effluent was adjusted to pH 1.5. The effects of acidity, sample flow rate and. volume, elution solution, and interfering ions on recovery of the two mercury species have been systematically investigated. Under optimal conditions, the limits of detection (LODs) were 72 and 44 ng L-1 for Hg2+ and MeHg+ (as Hg) with online concentration factors of 12.5 and 12.1, respectively. The relative standard deviations (RSDs) for nine replicate determinations at 5 ng mL(-1) levels of mercury species were 2.7 and 2.0% for Hg2+ and MeHg+, respectively. The calibration graphs were linear with a correlation coefficient of 0.9902 in the range of 0.5-100 ng mL(-1) for Hg2+ and 0.9976 in the range of 0.1-100 ng mL(-1) for MeHg+, respectively. The developed method was successfully applied to the direct determination of MeHg+ and Hg2+ in seafood samples, and the recoveries for the spiked samples were in the range of 89.9-102.4% (MeHg+) and 87.0-104.6% (Hg2+), respectively. The method was validated by analyzing a certified reference material DORM-2 (dogfish muscle), and the determined values were in good agreement with certified values.