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Soil strength sampling is both labor-intensive and time consuming. In most cases, soil strength sampling requires human labor in collecting samples and doing tests on-site in an unstructured environment or in a laboratory. The purpose of this research is to make an unprecedented attempt of establishing a fully autonomous soil strength sampling system. Two major issues were considered. 1. How to navigate a multi-sensor autonomous vehicle (mobile robot) in an unstructured environment. 2. How to measure the soil strength with an autonomous system. Along with the research, some basic issues including principles of motion control, mobile robot navigation strategies and soil strength sampling mechanisms were studied. A controller was built to synthesize the control information from multiple sensors. An autonomous soil strength sampling system was built, which can sample the soil in accord with principles of hand-held Duplex Cone Penetrometer (DCP). Measurements of the resistance from the autonomous soil sampler provide a reading that can be correlated to the California Bearing Ratio (CBR). An integrated GPS guided autonomous soil sampling robot was onstructed. The robot accomplished navigation to the targeted waypoints and measurement probing of soil strength during the on-site test for the Air Force Special Operations Forces in Florida. The significance of this research is that it is the first demonstration that a robotic solution for a tedious and potentially dangerous soil sampling task could be accomplished in an unstructured environment.