This study aimed to probe into the influence of implementing Web-quest supported science writing instruction on students' science learning and science writing. The subjects were 34 students in one class of grade six in an elementary school in Taiwan. The students participated in the instruction, which lasted for eight weeks. Data collection included Web-quest learning sheets, science writing learning sheets and interviews with students. The research findings demonstrated that Web-quest supported science writing instruction enhanced students' abilities to search by keywords, organize science contents and use scientific words. Students made significant progress in science writing and it revealed high and medium effect sizes. Changes in the students from the medium academic achievement group were the best and the difference was significant with a large effect size. (Contains 2 tables and 1 figure.)