Classroom-based achievement tests have been extensively used in Nigerian secondary schools especially after the introduction of continuous assessment in 1985. These achievement tests have been criticized by over the years for lack of proper psychometric properties of a test. These views bother on teachers' possession or non possession of competencies in test construction skills. This study developed and validated a Test Construction Skill Inventory (TCSI) for assessing the secondary school teachers' competencies in constructing classroom-based tests. Factor analysis was done on the 30-item instrument developed by the researchers. 25 items were found to be factorially valid. The TCSI was also found to be reliable with a coefficient of 0.73 and the secondary school teachers found almost all the 25 items important skills for quality classroom-based test construction. The TCSI was, therefore, recommended as an important measure for determining the secondary school teachers' test construction skill in Anambra State, Nigeria.