Despite the rising interest in the practice of formative assessment (FA) in language classrooms, research has rarely examined the factors that contribute to differential use of FA. The present study adopts an explanatory sequential mixed-method design to examine differences in FA use among Japanese junior and senior high school EFL (English as a Foreign Language) teachers. Based on Wiliam's (2010) model of FA strategies, quantitative analysis of a large-scale survey classified 727 participants into three levels (high, middle, and low) of FA use. In the qualitative follow-up phase, four selected survey respondents were interviewed and observed. The results indicated varying degrees of FA use among teachers, suggesting that the four FA variables (intentions, methods, purposes, and feedback) derived from Wiliam's model succeeded in identifying differential degrees of FA use.