Knowledge is not only an important cornerstone that constructs the cognitive ability in human beings, but is also one of the basic issues in intelligence science. With the development of intelligence science and technology, the study of knowledge uncertainty is attracting more and more attention. Knowledge uncertainty includes the inherent uncertainty of knowledge itself and the effect of external influence on knowledge. In the view of granular computing (GrC), the knowledge uncertainties of the fuzzy set theory model, the rough set theory model, the quotient space theory model, and some other related granular computing models are studied in this paper. The state-of-the-art and key issues of knowledge uncertainty are discussed.