The present study examined roles that mathematics plays in students' understanding of physical laws in science. Students in two classes of public junior high school received two types of instructions concerning the physical law predicting that weight of oxidized metal is proportionate to weight of metal before oxidation. Students under the experimental condition deduced the physical law from an atomic model first, and then designed experiments for themselves to obtain the proportion of weight of magnesium to weight of oxidized magnesium by using their knowledge about proportionality that they had learned in mathematics class. Students under the control condition induced the physical law from experiments of oxidation of magnesium. It was found that students under the experimental condition did better on tests about the calculation based on the physical law and took measurement errors into consideration more appropriately than students under the control condition.