Soil compaction is an important aspect of agriculture and different methods are available to measure this parameter. One method is called the cone index; measuring the force required per unit area for penetration of a standard steel cone into the ground. Cone penetrometers are used for cone index measurement. In this study, a tractor mounted, hydraulically-operated penetrometer was designed, built and evaluated. The penetrometer consisted of a frame, hydraulic driving unit, force transducer, depth measuring unit and data acquisition system. The entire penetrometer unit was mounted on the frame and held in place by the three-point hitch on a category III tractor. The hydraulic driving unit provided a constant penetration rate of 3 cm/s, in accordance with the ASAE S313.3 standard. A cantilever beam strain-gauge load cell measured the penetration force applied to the cone shaft and a photodiode sensor measured the penetration depth. A microcontroller-based data acquisition system was designed and developed to record and save the penetration data. Evaluations were conducted by comparing the data of the tractor- mounted penetrometer and that of a commercial hand-pushed penetrometer (Eijkelkamp). Analysis found no significant difference at the 95% confidence level. The penetrometer performance was found to be reliable and the unit's mechanical and electronic parts worked without malfunction.