We observed remarkable oscillatory forces with amplitude of 60-90 nN and periodicity of 3–4 water molecule diameters in an ambient environment between two silicon surfaces as the tip-sample distance decreased using the cantilever-based optical interfacial force microscope with lateral modulation. As the humidity increased, the oscillatory forces changed from being large at lower relative humidities, below 40%, to progressively becoming smaller as the humidity went higher than 40%. The frequency of oscillatory forces increased with the existence of lateral tip-modulation and with the slower approaching tip-speed. Its starting point distance has been found to originate from the topographic water structure.