A model for size-dependent surface energy of nanocrystals, free of any adjustable parameter, has been established based on a previous model for the size-dependent cohesive energy. The surface energy falls as the size of crystals decreases to several nanometers while the surface energy ratio between different facets is size-independent and is equal to the corresponding bulk ratio. The predictions agree with experimental or theoretical results for beryllium, magnesium, sodium, aluminum, and gold.