A novel method for measuring ultrathin (2–12 nm)SiO2film thickness is discussed. The process consists of: (1) formation of octadecyltrichlorosilane (OTS) self-assembled-monolayer (SAM) islands onSiO2of which thickness to be measured, (2) removal of theSiO2layers not covered by the OTS-SAM islands, and (3) measurement of the height difference between the etched and nonetched areas by atomic-force-microscopy. The OTS film is good resist against HF and its islands can be regarded as self-patterned-mask. Practical usefulness is demonstrated not only by the compatibility of the measured values but also by the short measurement period resulting from the directness of the method. ©1997 American Institute of Physics.