This paper aims at investigating the effects of a genre-based approach to teaching technical writing to Thai engineers, with a focus on writing work instructions, and the attitudes of Thai engineers toward this genre-based writing. The lesson plan was evaluated by three experts for the appropriateness of teaching technical writing to Thai engineers. Learning achievements have been examined using a comparison of pre- and post-tests for ten engineers who are in the same discipline, yet have different backgrounds of English proficiency. The learning outcomes were measured by the difference in scores between pre-test and post-test, and revealed that there was a significant difference between after teaching at p-value < 0.05 (p= 0.002) as indicated by their paired t-test analysis. Lastly, a satisfaction interview of the overall course has been conducted for four participants and showed that the engineers' attitudes were positive about the teaching method.