Traditional micro-sized and nano-sized YAG transparent ceramics were tested by nanoindentation at different peak loads. The micro-sized YAG transparent ceramics show a marked indentation size effect (ISE). However, for the nano-sized YAG transparent ceramics, the hardness was constant in the whole investigated range without any evidence of ISE. We show that the absence of indentation size effect for nano-sized YAG transparent ceramics can be accurately modeled using the plastic deformation mechanism of grain boundary sliding.