With regard to the implementation of Blended Learning (BL) in higher education, not much is known about the institution-wide participation status by the teacher, and ways to quantitatively assess such participation. This research first developed a framework for quantitatively assessing teachers' online participation in BL, and then evaluated it with data produced by 7272 teachers in 15,128 BL courses in six universities across China. This evaluation adopted a mixed methods design to investigate the intensity, regularity and interactivity of these teachers' online participation in BL. Three key findings emerged: 1) the framework effectively assessed the degrees and features of teachers' online participation in BL implementation, 2) these results advanced our understanding of BL adoption stages, and 3) the proposed framework proved to be technically adequate in assessing teachers' online participation in BL at an institutional level. Using this framework, institutions could gain a fuller picture of their BL implementation so as to refine their strategies and advance their BL agenda.