The purpose of this study was to examine the determinants of performance in arithmetic word problems. 210 sixth graders were given the word problems involving fractions, which had four sub-processes of problem solving: translation, integration, planning and execution. In addition, they took three tests:(a) existing knowledge test involving conceptual knowledge and procedural knowledge,(b) intelligence test for both verbal intelligence and numerical intelligence, and (c) reading span test. In order to predict the results of four sub-processes, the following variables were entered into a stepwise regression: conceptual and procedural knowledge, verbal and numerical intelligence standard score (ISS), and reading span. The regression revealed that the procedural knowledge and both verbal and numerical ISSs mainly predicted the performances of word problem solving. It also revealed that the conceptual knowledge determined the result of translation-process only, and reading span was not a significant determinant. It will be necessary to search for other conceptual knowledge being reluted to the whole process of problem solving.