The purpose of this quasi-experimental study was (a) to determine if there was a difference in knowledge achievement by Thailand's vocational students instructed using constructivist instruction (with open-end dialogue, problem-based learning, and cooperative learning) and using traditional instruction (with lectures, directed demonstrations, and scripted experiments); and (b) to identify whether Thai vocational students preferred instruction modeled upon constructivist philosophy rather than traditional instructional philosophy. The target population consisted of students participating in electronics courses. The sample was students who attended the Electronics Measurement and Instrument course at two selected technical colleges in central Thailand. Two intact classes at each college received both treatments (counterbalanced design). There were 108 students used as the sample in the study. An effect size and an ANCOVA were used to test the first research question. There was no statistical difference between the achievement scores of the students receiving constructivist instruction and those receiving traditionally instructed students. However, the constructivist-instructed students had higher scores on the posttest and the delayed post-test, compared to those of the traditionally instructed students. The samples mean scores, standard deviation, percentages, and opinions on surveys were used to compare results for the second research question. The findings showed that there was a significant difference in student preferences. Students preferred constructivist instruction.