The nonvolatile memory characteristics of metal-oxide-semiconductor structures containing Pt nanocrystals in SiO2 gate oxide were studied. The Pt nanocrystals of 2–3 nm in diameter were self-assembled from reduction of an ultrathin PtOx layer embedded in the SiO2 by vacuum annealing at 425°C. A large hysteresis loop was found in the capacitance–voltage (C–V) relation indicating this significant memory effect. However, two different charge storage mechanisms were found for the Pt nanocrystals in devices with different tunnel oxide thickness. A counterclockwise C–V hysteresis was induced from substrate injection for the devices made with a thin tunnel oxide layer 2.5–5.0 nm thick. Contrast, a clockwise behavior attributed to the electron transfer from charged defects in the gate oxide was found for the devices having a tunnel oxide layer 7.5 nm thick. The relatively stable memory characteristics of Pt nanocrystals resulted from substrate injection were also demonstrated.