The aim of this study was to construct and validate a mathematics achievement test for primary six school pupils, Four research questions were employed in constructing the study. The design of the study is instrumentation research design. Using random sampling technique via balloting 10 primary schools was drawn from 54 primary schools in Obio Akpor L.G.A. Through stratified random sampling technique 858 primary six pupils were drawn from the population of 2928 primary six pupils from Obio/Akpor L.G.A. The Form 1 MAT was face-validated in terms of clarity of words and plausibility of distracter by specialist in Educational Measurement and Evaluation and mathematics teachers. The Form II MAT consist of what was adminstered to 858 pupils and their response were used in determining proportions of pupils who showed mastery ability.The findings of this study show that the MAT is a valid and reliable instrument for measuring achievements in mathematics tests.