This book shows the effect of constructivist method in students' mathematics achievement in large classes. It is prepared to unveil the critical issues about the notion that constructivist approach is the best means in the teaching-learning process for teaching effectiveness. It is a research-based book. It contains the whole parts of a completed research. The findings that are stipulated in the study may help students to clearly learn secondary mathematics, attractively promote students' retention in the important mathematical concepts, and inspire students to pursue higher education and to be progressive persons; encourage math teachers to use varied teaching methods to suit the needs of the students; and convince school officials and other educative personalities to effectively plan for suitable class size which contributes to the academic performance of the students.