Lexical bundles, as fixed-form recurrent word combinations of multiple words (Biber, Johansson, Leech, Conrad, & Finegan, 1999), have been documented to play an important role “in the fluent linguistic production and a key distinguishing feature of particular modes, registers and genres” (Hyland & Jiang, 2018, p. 383). A number of studies have found that second language learners experience difficulties while using bundles by misusing, overusing or underusing these bundles in their writing (Ädel & Erman, 2012; Hyland, 2008b; Wei & Lei, 2011). However, despite an extensive number of studies conducted on the use of lexical bundles in various fields, there is still paucity of research into the use of such linguistic devices across various disciplines, particularly in the Iranian context. Therefore, by adopting a corpus-based analysis approach, this study explored the frequency, functions and structure of 4-word lexical bundle use across four different disciplines (business and tourism as soft sciences, and mechanical engineering and civil engineering as hard sciences, see Hyland, 2008b) in the master theses of native English-speaking writers and Iranian L2 writers. For this purpose, two corpora, each containing 60 master theses totaling 120, were selected. The findings of the study revealed that, in total, Iranian writers incorporated more lexical bundles in their texts than their native English counterparts and that its use varied functionally and structurally across not only native and nonnative corpora but also across disciplines (soft and hard sciences). The study has important implications for Iranian L2 writers of the respective disciplines as well as genre-based instruction in English for both academic and specific purposes.