The study investigated the effect of Realia Instructional Method (RIM) on students' academic achievement in four-stroke-engine-operation in technical colleges in North-West Nigeria. The study was guided by two objectives, two research questions, and two null hypotheses. The study anchored on theory of constructivism and social-constructivist theories of learning. Pretest-posttest experimental design was used for the study. The population consists of 480 technical colleges students in SS1 Motor Vehicle Mechanics Work (MVMW) in 2020/2021 academic session in north-west Nigeria. Two intact classes in two technical colleges consisting of a total of 75 SS1students (37experimental group and 38 control group) were randomly selected as sample for the study. Pretest, Posttest achievement test were used as instrument for data collection. The instrument contained 40 multiple choice test items with reliability coefficient of 0.85 obtained using split half method. Data collected was analyzed using mean and standard deviation and t-test was used to test the two null hypotheses at 0.05 level of significance. The result revealed among others that there was a significant difference in the use of realia and conventional method, in favour of the realia instructional method. It is therefore recommended that teachers should be using RIM in teaching four-stroke-engine-operation in technical colleges and MVMW in general. This will encourage teachers to cover all parts of the curriculum. School administrators should encourage the shift from theory based/lecture method of teaching to practical through the use of Realia Instructional Method.