In the case of quantitative determination of yellow phosphorus according to silver phosphonium formed used to be oxidized with nitro-hydrochloric acid (aqua regia). On adding small quantity of concentrated sulfuric acid into this reaction, we can save time and escape from the unpleasant phenomen which takes place duriug the further operatiov, because chlorine or hydrochloric acid is entirely expelled by evaporation. And no wrong influence owing to this modification upon the results of determination was found. We can determine the quantity of yellow phosphorus twice by the same material by each different method, that is, determination by titration, and by gravimetry. In other words, in the quantitative determination of yellow phosphorus for legal purpose, we believe that we could reduce the time of operation, and control the results of the first measuring method through the other method.