Distortion of diatom assemblages by differential dissolution has long hindered their utilization for paleoceanographic research. We propose a quantitative index defined as the ratio of number of complete valves/number of complete copulae (V/C ratio) to evaluate dissolution levels of Neodenticula seminae. We observe that an almost equal number of valves and copulae of this species occur in nearly undissolved assemblages from the water column. In spite of this, copulae are dominant in surface sediments, indicating differential dissolution between these frustule components. To confirm this, we carried out an experiment and found the copulae to be more dissolution tolerant than the valves. Based on these results, valve number in any assemblage prior to the start of dissolution is predicted with an expression using; 1) the number of both components after dissolution and their ratio; 2) the ratio of both components before dissolution starts (1.0) and 3) the ratio of dissolution tolerance between both components (shown experimentally). In short, we can obtain the quantified N. seminae dissolution level by a comparison of dissolution tolerance between both frustule components in experiment, and by counting both components in a sample. Neodenticua seminae is among the most important primary producers in the mid to high latitude North Pacific during the Quaternary. Thus, we strongly stress that the application of this index will allow us not only to revise opaline dissolution history, but to take the more quantitative step in reconstructing paleoproductivity and/or mass balance of opal during the Quaternary.