  The spectroscopy of the apparent horizon of Vaidya black holes is
investigated via adiabatic invariance. We obtain an equally spaced entropy
spectrum with its quantum equal to the one given by Bekenstein. We demonstrate
that the quantization of entropy and area is a generic property of horizon, not
only for stationary black holes, and the results also exit in a dynamical black
hole. Our work also shows that the quantization of black hole is closely
related to Hawking temperature, which is an interesting thing.
