Abstract: An extensive experimental-mechanistic study was conducted to reveal the relationship between the light weight deflectometer (LWD) measured deflections and the degree of compaction of pavement base materials. Both laboratory experiments and test pits experiments were performed with different types of pavement base materials. The modulus based maximum allowable LWD deflections under different structural and compaction conditions were developed for the most commonly used pavement base aggregate in Indiana. The maximum allowable deflections are based on the equivalent subgrade modulus and the thickness of the layer to be compacted. It is emphasized that the LWD deflections must be measured as soon as the material is compacted or before the moisture content decreases beyond a specified range. Therefore, the maximum allowable deflections are specified in terms of the difference between the actual moisture content and the optimum moisture content. The maximum allowable deflection values provide a sound basis for compaction quality control using LWD devices.