This exploratory research presents the implementation and evaluation of the effects of integrating corpus consultation with business English writing instruction. The subjects consisted of English as a foreign language (EFL) learners enrolled in two undergraduate business English writing classes. Two groups of EFL students were randomly assigned, one group (n = 49) receiving corpus-based writing instruction constructed on a Moodle course management system (CMS), while the other group (n = 58) was given traditional lecture-based instruction. A mixed methods design combining qualitative and quantitative approaches has been chosen to investigate the overall effect of the corpus-based intervention on the improvement of business letter writing performance in aspects of lexical and syntactic complexity, as well as learners' perceptions. The comparison of the pre- and post-tests of writing revealed a significant difference between the experimental and control groups after the instruction. Significant differences in students' lexical and syntactic complexity were found between the pre- and post-test of the experimental group. Further, in response to a questionnaire survey and interview, the students stated they improved their writing skills regarding vocabulary, syntactic structure and content in general, and their writing confidence and linguistics awareness were also enhanced. The results suggest that the corpus provides useful resources to supplement existing materials.