The structure of nanometer-sized strained Ce islands epitaxially grown on a Si substrate was studied using ultrasonic force microscopy (UFM), which combines the sensitivity to elastic structure of acoustic microscopy with the nanoscale spatial resolution of atomic force microscopy. UFM not only images the local surface elasticity variations between the Ge dots and the substrate with a spatial resolution of about 5 nm, but is also capable of detecting the strain variation across the dot, via the modification of the local stiffness.