This study bases itself on the conceptions of cognitive linguistics. It aligns with the perspective of the Creative Construction Hypothesis in examining learners’ L2 production in EFL writing. Evidences from the research reveal that in order to organise and output the intended messages, learners draw upon their repertoire of both L1 and L2 knowledge and other non-linguistic elements to accomplish the writing tasks. This confirms Corder’s (1981) view that the so-called “errors” are but learners’ idiosyncratic dialects manifesting their active choices and decision-making during the process of meaning construction and their efforts of approximating the target language patterns, rather than “passive” habits of L1. A "written production model" inspired by Levelt’s (1993) speech production model, is created to illustrate the information process in learners’ language production. Five categories of information processing tactics are identified that account for the learners' production of idiosyncratic dialects.