This study examines whether formulaic sequences might be stored and retrieved as wholes in a holistic fashion rather than analytically. Another goal is to see whether the processing of L2 sequences is affected by L2 proficiency, grammaticality and length of expressions. Grammaticality judgment task was conducted and data was collected from Korean L2 English learners of low- and intermediate-level of proficiency. First, results showed that formulaic sequences were responded more rapidly and accurately than nonformulaic sequences. This provides additional support for the claim that formulaic sequences are stored and processed as single lexical units even in L2 speakers` mental lexicon. Second, the formulaicity effect was constant at both levels, indicating that formulaic sequences are acquired and processed from the early stage of L2 development. Finally, it was found that grammatical sequences were processed more quickly and accurately than ungrammatical ones and that shorter expressions were judged faster and more accurately than longer ones, implying more efficient processing and less memory load for shorter and grammatical expressions. Building on the results in the present study, we propose a formal account for the quicker and accurate processing of formulaic sequences with reference to psycholinguistic model and processing constraints.