This study was designed to determine the effect of reflective inquiry instructional technique on achievement of students in Technical Colleges. The study adopted a pre-test, post-test, non-equivalent control group, quasi-experimental research design which involved groups of students in their intact class assigned to experimental group and control group. The population of the study was 105 Tech II students of and Electronic Works trade in Technical Colleges in Lagos State. Four research questions and five null hypotheses, tested at 0.05 level of significance, guided the study. The instruments used for data collection were Electronic Works Achievement Test (EWAT). To ensure content validity of the EWAT, a table of specification was built for the test. The reflective inquiry lesson plan, EWAT and the training manual were subjected to face validation by three experts. The EWAT was trial-tested to determine its psychometric indices and reliability coefficient. The trial test for determining the coefficient of stability of the EWAT was carried out using test re-test reliability technique. Pearson Product Moment Correlation coefficient of the EWAT was found to be 0.83. Mean was used to answer the research questions; while ANCOVA was employed to test the hypotheses. The study found out that reflective inquiry instructional technique is more effective in improving students' achievement in Electronic Works trade than conventional method. There was an effect of gender on students' achievement in Electronic works trade favouring boys. The study found out that there are no interaction effects of reflective inquiry instructional technique and gender on achievement of students in Electronic works trade. This simply means that the effectiveness of reflective inquiry instructional technique on students' achievement in Electronic works trade does not depend on gender. The study found out that there was an interaction effect of students' gender and their ability with respect to their mean scores on Electronic works trade achievement test. This simply means that reflective inquiry instructional technique is more effective in improving students' achievement in Electronic works trade regardless of gender or ability level. Consequently, it was recommended that The National Board for Technical Education (NBTE) should carry out a review of Electronic works trade curriculum for Technical Colleges with a view to incorporating the reflective inquiry instructional technique into the teaching of Electronic works trade.