The technique of an on-line preconcentration by the direct sulfide precipitation has been developed. Sn was homogeneously precipitated by sulfide, which was generated in situ from the hydrolysis of thioaceteamide. Precipitate was collected on a filter in the line and dissolved out instantaneously by KOH to be sent to an ICP. The enrichment factor was 4 with the sampling speed of 15/hr for 1.0 mL of sample. It was increased to more than 40 times when the sampling volume was increased to 10 mL with the sampling speed of 5/hr. $Sn^{2+}/Sn^{4+}$ could be separately determined with the on-line precipitation technique. The method was applied to the analysis of NIST SRM 1566 Oyster sample and yielded good agreement with the certified value.