This study aimed at investigating the effect of diagnostic testing on students' academic achievement in secondary school quantitative economics. In conducting the study, 3 research questions and 3 stated hypotheses were answered. The study is quasi-experimental employing 2x4 factorial pretest-posttest design. The sample consisted of 210 Senior Secondary 3 (SS3) economics students in the four co-educational schools purposely selected from Nnewi Education Zone of Anambra State in Nigeria. They were allocated to 3 experimental groups and 1 control group. Students' responses to two instruments titled Diagnostic Quantitative Economics Skill Test (DQEST) and Test of Achievement in Quantitative Economics (TAQE) constituted relevant data for the study. Instruments for data analysis were t-test and ANCOVA. Results of the analysis indicate a significant effect of treatment on students' achievement in favor of DQEST with feedback and remediation group only (F (3, 209) = 22.3114, p > 0.05). Gender made no significant difference on students' achievement in TAQE. Thus, diagnostic tests are effective when used with feedback and remediation. The use of DQEST with feedback and remediation in teaching and learning of quantitative economics is therefore recommended.