We studied to develop a certified reference material of seawater for nutrient analysis whose relative combined standard uncertainty for dissolved silicate was < 0.5 %. In order to precisely measure the content of dissolved silicate in seawater, widely used absorption spectrometry with the so-called molybdenum blue was investigated as one of the highly sensitive determination methods. For the absorption spectrometry, a batch mode and a continuous flow analysis (CFA) mode were used and the characteristics of the calibration curves in the both modes were examined in detail. The calibration curve in the batch mode could be explained by a first-order regression line, whereas that in the CFA mode fitted to a second-order regression line. Therefore, for measuring the silicate content of seawater whose matrix depends on its origin, a standard addition method in a batch mode is preferable, though strict control of the compositions of the solutions for the standard addition calibration curve are greatly important for getting good linearity of the curve. In addition, in the case of the standard addition method, it is important to measure the blank value including the background; in the present study, the origins related to the blank were discussed and evaluated. The relative combined standard uncertainty of 0.48 % could be achieved for the measurement of content of 28 mu mol/kg silicate in seawater, providing that the volume ratio between each of the molybdenum blue colorimetric reagents and the seawater was kept constant among all the solutions for the standard addition calibration curve. The procedure was validated using artificial seawater with a known concentration of silicate. Such highly improved precision and trueness which reaches the recent target of the community related to analysis of nutrients in seawater is reported for the first time. The developed procedure will provide a good way of validation for the analysts who need measurement with high precision and trueness on the content of silicate in seawater and other samples.