The quasi-experimental study utilized an interrupted time-series design to examine the effect of 20 third graders' writing achievement as a result of a trait-based instructional approach to writing. The primary researcher provided writing professional development on traits-based instructional and assessment for a third-grade team. One of those teachers agreed to participate in the study. Nearly 200 writing samples were independently scored to establish a pre-test baseline, and a post-test baseline to investigate the effects. The pre-test trend was slightly negative, and the post-test was 23% higher and showed a more positive trend. A paired samples t-test indicated a statistically significant increase and the effect was large (d=2.38). Implications for instruction and limitations of the study are discussed.