Written expression in science is viewed as an essential component of scientific literacy. This study reports on the process of developing an instrument for assessing students’ ability with regard to writing about science topics, and the statistical results of the research conducted in Taiwan with fourth and sixth graders. The Written Expression in Science instrument includes four components: understanding science vocabulary, identifying scientifically-oriented questions, providing a proper method or evidence in responding to questions, as well as formulating and connecting explanations based on evidence. A combination of CTT-based and IRT-based approaches was used to analyze the assessment data. The data analysis indicates that the instrument developed in this study has satisfactory validity and reliability measures. Data from the Written Expression in Science instrument reveal three major findings: the items of the instrument are slightly difficult for middle level students; the wide range of item difficulty estimates and the flat shape of the TIF curve suggest that the instrument is appropriate for a relatively large proportion of students with higher ability in science writing; and multiple regression indicates that the students’ writing proficiency on science topics was associated with aspects such as prior knowledge of science and Chinese language proficiency. Based on the relationships among science, language and literacy, integrating explicit writing instruction into inquiry-based instruction is highly recommended.