This study aims to examine the effects of the process genre approach  (PGA)  on English foreign language (EFL) upper secondary students’ writing abilityin terms of content, organization, vocabulary and grammar.The quasi-experimental  design named the nonequivalent control group design with pretest and posttest was usedto collect quantitative data. Participants including 25 English-majored students of grade 10 at a specialized upper secondary schoolin Vietnam divided into two groups: one control group (12 students) and one experimental group (13 students).T-Test results of pretest and posttest scores revealed that the group treated with the process genre(PG) instruction significantly outperformed the control group in writing ability in terms of content, vocabulary, language (grammar) except for organization. The findings of this study suggested that the PGA should be used as pedagogical tool to teach writing for EFL upper secondary students in Asian contexts.