The purpose of the present study is to investigate the impact of learners' L1s and proficiency levels on their written production. This study also examined the influence of speech upon their writing. The following research questions were explored: (a) How do L1 and proficiency levels of learners affect their degrees of register awareness? (b) Which linguistic features distinguishing writing and speech registers are characteristic of each Asian learner group? This study draws on four sub-corpora of the International Corpus Network of Asian Learners of English (ICNALE), which is considered to be the largest East Asian composition database. Using the methodology originally developed by Biber (1988) to analyze the differences in the spoken and written registers of English, this study investigated the differences in a wide range of linguistic features among Asian learners of English. The results suggest that the L1s of learners affect the degree of their register awareness. Hong Kong learners display a set of stylistically appropriate features, such as nominalizations, predictive modals, and conjuncts, in their academic prose whereas Japanese learners exhibit many of informal features, such as first person pronouns, private verbs, and independent clause coordination, in their written production. Besides, Korean and Taiwanese learners show several features typical of speech, including second person pronouns, in their writing. In addition, this study demonstrates the effectiveness of Biber's list of linguistic features in the study of spoken nature in L2 writing.