Generation 1.5 ESL students are often characterized as achieving quite advanced oral/aural proficiency but less developed academic literacy (Forrest, 2006; Singhal, 2004). And yet little is known about the relationship between their spoken and written discourse. Thus, this paper discusses a case study that explored the relationship of spoken and written discourse of a Generation 1.5 ESL student, whom we call Mary, in a college ESL composition classroom. The findings revealed that Mary employed speechlike features for her writing (i.e., use of colloquial language and lack of explicitness in linguistic and content terms). These findings indicate that Mary seemed to write the way she spoke, which contributed to making her writing informal, implicit, and less persuasive. In addition to examining the language features, we explored the effect of explicit instruction on the difference between spoken and written discourse. Such instruction turned out to be partially effective. Findings arising from this study suggest that Generation 1.5 ESL students such as Mary employ two approaches (i.e., writing/speaking and reading/writing connections) to improving writing.