Round guardrail posts may provide an important value-added option for small-diameter thinnings (SDTs). Such posts require minimum processing and are believed to have higher strength for the equivalent rectangular volume. A modified version of the Midwest Guardrail System (MGS) was developed, tested, and evaluated utilizing small-diameter, round wood posts in lieu of steel wide-flange posts. This system has been accepted for use on the national highway system by the United States Department of Transportation, Federal Highway Administration. The barrier system was modified using three different timber species - Douglas Fir, Ponderosa Pine, and Southern Yellow Pine. Barrier VII computer simulation, combined with cantilever post testing conducted both in a rigid sleeve and in soil, were used to determine the required post diameter for each species. The final recommended nominal sizes were determined. A grading criteria limiting knot size and ring density was established for each species. The minimum ring density for each post species was to equal or exceed 6 rings-per-inch (rpi) for Douglas Fir, 6 rpi for Ponderosa Pine, and 4 rpi for Southern Yellow Pine. Two of the guardrail systems, one using Douglas Fir posts and another using Ponderosa Pine posts, were full-scale vehicle crash tested and reported in accordance to the Test Level 3 (TL-3) requirements specified in the National Cooperative Highway Research Program (NCHRP) Report No. 350. Crash testing of the third system, using Southern Pine posts, was not conducted based on the prior successful testing of a standard W-beam guardrail with 184-mm (7.25-in.) diameter, Southern Yellow Pine posts as well as comparable post design strength to that of the other two species. The two full-scale crash tests showed that the modified MGS functioned adequately for both wood species. Three round wood post alternatives are recommended as an acceptable substitute to the standard W152×l3.4 (W6×9) steel post utilized in the MGS.