Reflection in education has seen an upsurge in research interest since as reported by Dewey (How we think, Prometheus Books, Buffalo, 1991) ground-breaking work, which plays an integral role in fostering students' self-reflection and demonstrable learning skills. However there have not been enough studies to determine whether writing instruction from the perspective of self-reflection before action improves undergraduate students' essay writing achievement. This article investigates the effects of Ethiopian university-undergraduate students' self-reflection before action on their essay writing achievement through the mediation of teaching writing instruction. The study's methodology involved two phases of intervention in a counterbalanced design. The participants were 47 male and female students aged 19-22, from Arba Minch University. They were assigned to a control group and an experimental group using the matched sampling technique. During the first phase, the experimental group (Group A) was instructed to engage in self-reflection-based course activities over a period of two months while the control group was not. In the second phase Group B was engaged in self-reflection-based course activities over a period of two months while Group A was kept under control. To collect data, preliminary students' self-reflection was provided using an open-ended questionnaire, and pre- and post-tests of writing were given to both groups. Mean and standard deviation statistics were applied after the thematic analysis of the qualitative data. Additionally, a univariate analysis of variance was performed to answer the query regarding the impact of self-reflection on students' essay writing performance. The findings demonstrated that, despite the students encountering numerous difficulties during their essay-writing exercises, the self-reflection-based instruction had a significant positive impact on the participants' essay-writing achievement. In conclusion, the findings have some implications for teaching essay writing instructions.