The purpose of this study is to determine the effects of left-brain, right-brain, and whole-brain dominance on students’ English writing proficiency. Students were evaluated in terms of (a) overall writing proficiency (total score), as well as (b) five sub-skills that compose the writing proficiency (contents, organization, vocabulary, language use, and mechanics). The hypothesis was that students would not show any differences in either (a) the overall writing proficiency or (b) its five sub-skills, regardless of brain dominance. The experiment was conducted with 40 university students in 3rd year of an English major. A writing test and a Brain Preference Indicator (BPI) test were administered. The BPI test categorized the students into 3 groups: left-brain dominance (LBD), right-brain dominance (RBD), and whole-brain use (WBU) meaning a balance of left-brain and right-brain. The ANOVA result showed that there was no statistically significant difference among the three groups in overall writing proficiency. However, the MANOVA result showed that there were statistically significant differences between the three groups in some of the five sub-skills of English writing (hypothesis b). That is to say, WBU groups outperformed both LBD and RBD in organization and vocabulary. In conclusion, the results indicate that certain writing sub-skills, organization and vocabulary, are significantly influenced by brain dominance.