A number of studies have suggested the possible benefits of using concordancers in English as a Foreign Language (EFL) writing. To date, however, there is still limited understanding of how EFL writers may make use of such tools along with dictionaries when engaging in a writing task. To better understand the question, this study investigated a class of non-English-major college students (N = 14) over a semester. Four online corpus tools, including monolingual and bilingual concordancers and collocation retrieval systems, were provided, along with two online dictionaries. After two tool-training sessions, students performed three timed-writing tasks online during three consecutive months and received individual stimulus recall interviews after each writing task. The recall interviews served as the main source of data; other data included the video clips of the writing process, student writing samples, and the researchers' notes. The result showed that students used corpus tools and the bilingual dictionary for different purposes. They tended to use a bilingual dictionary when information on word form and word meaning was needed. When searching for information related to word usage, collocation information, and grammar patterns, they chose corpus tools more often than a bilingual dictionary. However, they also turned to corpus tools for meaning and form when the bilingual dictionary failed to provide clear word meanings, when they needed to search for word strings, and when they needed to just confirm an intuition regarding either word form or word meaning. This study provides empirical data that help to better understand how corpus tools contribute to EFL writing.