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This study is aimed for investigating (1) the accuracy of both linear methods on classical test theory approach and mean &amp; mean method on item response theory approach in vertical equitting; (2) the characteristic of dikotomous item test on reasoning test of plane figure area conservation for grade IV, V and VI elementary school; and (3) the reasoning development of plane figure area conservation on the students (θ) in grade IV, V and VI based on the developed test instrument. Vertical equitting in this study use anchor item design on non-equivalen group. Standart Error of Equating (SEE) on linear method and Root Mean Square Different (RMSD) on mean &amp; mean method is resulted by 0,215 and 0,354 respectively from the vertical equitting. The difference on the value of both SEE on classical approach and RMSD on IRT approach is quite small from which suggest that its meaningfulness should be analyze more dephtly. This study suggest that all of three instrument tests provide threshold value (b) or item difficulty level of medium category (-1 < b < 1) with threshold average 0,150, - 0,016, and 0,272 respectively and good discriminant or slope (a) (a > 0,4) with slope average 0,781; 0,750; and 0,881 respectively. Furthermore, the calibration results show that, after equitting is conducted, there is improvement on the proportion of students on grade IV, V, and VI ability average with θ-value -0,124; 0,414 and 0,054 respectively.

vertical equating, reasoning test of area conservation, classical test theory, item response theory