Charge trapping properties of Au nanocrystals (NCs) with various sizes and densities embedded in metal oxide semiconductor (MOS) were investigated. Comparing with the Nc size, experimental results show that the NC spacing is more influential in electron trapping. Au NCs with the size of 8~13nm have larger memory window than the Au NCs of 13-17nm and 6-8nm, which can be ascribe to the higher field-enhancement effect caused by the larger NC spacing. Optimized NC size can improve the specific characteristic of memory device effectively.