Size modification of Au nanoparticles （NPs）, deposited on the Au-thick film surface, induced by highly charged ions （HCI） 40Ar12+ with fixed low dose （4.3×1011 ions/cm2） but various energyies （ranged from 146.64keV to 290.64keV） at room temperature （293.15K）, was investigated by atomic force microscopy （AFM） and transmission electron microscopy （TEM）. The selected 40Ar12+ beams were provided by electron cyclotron resonance source （ECRIS） of the National Laboratory of the Heavy Ion Research Facility in Lanzhou （HIRFL）. The morphological changes of NPs were interpreted by collisional mixing model, Ostwald ripening （OR） model and inverse Ostwald ripening （IOR） model of spherical NPs on a substrate. Theoretical and experimental studies both prove that there exist the critical energy E*, for low ion energy （EE*）, the mean diameter R of NPs increases with the increasing energy of incident ions E, while for higher projectile energy （E>E*）, it decreases with increasing energy.