The study investigates interaction effects of formative testing and attitudinal types as they affect students' achievement in mathematics. A total of 120 students are drawn from the three geopolitical zones of Delta State and participated in the experiment. Three-of-the-four formative test instruments were used namely: self-report rating scale; quadratic achievement test, graphical solution of Quadratic equation achievement test, and graphical solution of quadratic equation ability instrument to aid data collection for the study. Data were analyzed using mean, standard deviation and two-way analysis of variance. Results shows that formative test (with reference to close book formative testing) contributed significantly to students' achievement in mathematics. Close book, followed by open book formative testing type are found to significantly contribute to students' achievement. Implications of the findings will help to boost the quality of teaching and learning of science (with respect to mathematics) education in Nigeria such as yield a useful direction towards the development of mathematics education in relation to learners' achievement as well as give a picture of the importance of formative testing to performance achievement mathematics.