The purpose of this study is to examine the effects of science writing heuristic (SWH) instruction based on Tolumin`s structure of argument, on students` science process skills and scientific attitude. In the instruction, unstructured problems were selected with issues to be argued, in which interactions among students are stressed. This research involved 58 students of 5th grade at an elementary school, and divided them into an experimental group with 29 and a control group with 29. The former was treated with instruction using argumentation as SWH, while the latter with conventional ways of instruction. Science process skills and scientific attitude were compared between the two groups and within each of the groups. The results implied that science instruction using science writing heuristic is effective in improving learning science process skills and scientific attitude.