There has been much research on strategic planning (i.e., a period of time given prior to a task) and on-line planning (i.e., active engagement in formulation and monitoring on task), under the premise that language acquisition would be enhanced in such planned conditions by reducing cognitive load. However, the underlying mechanisms of this rationale have not been fully understood. To develop this present understanding further, this theoretical paper aims to explore the psycholinguistic mechanisms of strategic and on-line planning effects on L2 performance. Drawing on pedagogic recommendations for focus-on-form, the paper argues that strategic planning and on-line planning may promote incidental focus-on-form but to different extents. In particular, it is suggested that on-line planning, beyond a simple improvement of accuracy, would increase consciousness of form and bring learners into deeper grammar processing. In language classrooms, it is of significance to consider how to implement strategic and on-line planning in authentic ways.