This paper uses the compatible probability functions to define the notion of upper entropy and lower entropy of a belief function as a generalization of the Shannon entropy. The upper entropy measures the amount of information conveyed by the evidence currently available. The lower entropy measures the maximum possible amount of information that can be obtained if further evidence becomes available. This paper also analyzes the different characteristics of these entropies and the computational aspect. The study demonstrates usefulness of compatible probability functions to apply various notions from the probability theory to the theory of belief functions.