Abstract The study was an experimental endeavor to explore the use of an online bilingual dictionary in EFL writing by gauging its effect on writing scores, assessing its impact on lexical sophistication, and examining learners’ dictionary lookup behavior during writing. A class of EFL students (n=34) at a Chinese university were asked to write two English compositions, one without access to a dictionary and the other with the help of Bing.dict, one of the most popular online bilingual dictionaries in China. A screen recorder was installed to capture in real time the whole process of dictionary consultation and writing. Results indicate that the use of Bing.dict exerted a significantly negative effect on the overall composition scores and the component scores for content and language use, although it did help to increase the students’ lexical range as measured by the Lexical Frequency Profile. A variety of dictionary-based errors were committed in terms of lexicon, syntax and collocation due to the students’ inadequate dictionary use skills and the unsatisfactory quality of the dictionary for productive language use. A close examination of the screen recordings reveals that the students adopted a range of poor consultation strategies during writing. They were content to find an English equivalent to a Chinese lexical item without bothering to further check its meaning and usage. In addition, they tried in vain to find ready text translations for Chinese sentences or sentence fragments. Focusing on the unreliable web-crawled examples, very few of the students paid attention to the source dictionaries. Besides, most of them were prone to accept what was offered in the dictionary without independent thinking. The findings of the study show a rather disappointing picture of the use of an online bilingual dictionary for language production, suggesting that it is absolutely imperative to train EFL learners on dictionary use.