A nanoindenter apparatus is developed to measure the microhardness and microviscoelastic properties (in compression) of extremely thin films. In-situ indentation measurements are made by polarization interferometer by monitoring the absolute motions of the sample and indenter. A linear actuator provides the load, and the indenter load is inferred from the position of the indenter measured by the interferometer and the stiffness of the indenter parallel spring guide. A personal computer and associated electronics provide the control for load and penetration. These allow the instrument to determine microhardness and characterize microviscoelastic creep and relaxation properties. The linear actuator and the sample parallel spring guide are supported by an air bearing stage which is translated at a constant speed to conduct scratch tests for adhesion measurements. Based on the data reported in the paper, we find that microhardness and microviscoelastic properties at extremely low loads (or penetrations) are load [or penetration) dependent. © 1988 by ASME.