Abstract: The current study, based on the observation of the uses of modal verb may in English, establishes an analytical model for the grounding function of modal verb may, within the framework of grounding theory in Cognitive Grammar.The author proposes that modal verb may, as a grounding element, imposes a momentum of assumption, prediction, imagination, etc.for the evolution of the process profiled by the may-sentence into a possible future-oriented process.It is observed that the reality possibility of the process profiled by the may-sentence varies from high, medial to low possibilities.The variation is motivated mainly by the cognitive momentum of may, and related to the conceptualizer's modality of assumption, prediction and imagination.