Others have identified three normal operating regions for silicon-on-insulator (SOI) MOSFET's. In two of these regions the threshold voltage depends on the silicon film thickness and the buried insulator thickness. Based on the threshold equations, a method has been developed to nondestructively measure the two film thicknesses. The method uses a feedback amplifier to hold the drain biases nearly constant while the body and/or the buried gate voltages are varied. Calculated threshold voltages from the top-gate voltages are used to calculate the film thicknesses. The method is illustrated on devices built in oxygen implanted substrates. Measurements compare well with SEM image measurements.