This study investigated the effectiveness of explicit grammar instruction about sentence structure within a writing program for ESL Year 5/6 primary school students. Twenty students were recruited to participate in the study. The control group consisted of ten students who were randomly assigned by class group to a whole writing program. They did not receive teaching intervention about grammar. The remaining ten students - the experimental treatment group - were randomly assigned by class group to an explicit grammar intervention group. The students in the control group received twelve 45 minute lessons [two per week]. These lessons included descriptive writing of imaginary people and factual places without grammatical instruction. The students in the intervention group received explicit teaching about prepositional phrases and dependent clauses within twelve 45 minute lessons of the same descriptive writing program given to the control group. To assess the effectiveness of the explicit grammar intervention, pretreatment and posttreatment descriptive writing samples were collected from the students. This has been administered successfully in other studies as it can yield useful about the semantic and syntactical development of the writers according to their age and grade. This study also sought to investigate the effects of an L2 background on development of written syntax. Six hypotheses examined the effectiveness of the treatment intervention in expanding sentence structures in descriptive writing. Two-way analysis of variance [ repeated measures] design was employed in the statistical interpretation of the data gathered. The results of this study supported the hypothesis that explicit grammatical teaching has an effect on written language performance with an increase in the use of prepositional phrases, dependent clauses, adjectives and text length. However, the data indicated that there was not a significant difference, posttreatment, between the experimental treatment group and the control group with regard to their use of adverbs and increase in clause length. Suggestions for further research are outlined, and implications for the syntactical development of ESL writers in relation to specific procedures for teachers of ESL students are recommended in the conclusion of this study.