Central to argumentative writing is the proper use of epistemic devices (EDs), which distinguish writers' opinions from facts and evaluate the degree of certainty expressed in their statements. Important as these devices are, they turn out to constitute a thorny area for non-native speakers (NNS). Previous research indicates that Chinese EFL learners differ significantly from the native speakers (NS) in marking epistemic modality. One problem of previous studies is that the essay topics are not well controlled, which makes it somewhat ambiguous as to whether the observed linguistic discrepancies are caused by the NNS/NS difference or by the topic differences. This paper sets out to explore much more comparable data from International Corpus Network of Asian Learners of English (ICNALE). The results show that while both NS group and NNS groups are heavily dependent on a restricted range of items, the manipulation of epistemic modality is particularly problematic for the L2 students who employ syntactically simpler constructions and rely on a more limited range of devices, as already discovered in the previous studies. Nevertheless, this study also shows that the most proficient L2 students modify their statements with less certainty markers and more tentative expressions than do their L1 counterparts, and that all learner groups, regardless of their overall language proficiency, use less boosters than L1 writers, which is in sharp contrast with previous studies. The ability to mark epistemic modality has much to do with L2 proficiency. While lower-band students exhibit a heavy reliance on a narrower range of items for strong assertions, higher-band students tend to be more tentative and demonstrate a more native-like use of some Eds. The observed patterns are explained in the light of the inherent properties of English EDs, the imperfect modal instruction and learner factors.