Abstract: Traditional T-bar sounding probe section is circular, for two extreme probe surface conditions: completely smooth and completely rough, the penetration resistance difference is 26% of the moderate roughness; penetration resistance is strongly influenced by the surface roughness. In order to reduce the influence of the surface roughness on the probe resistance, oval cross section probe is studied. Assume that soft soil strength obey the Tresca strength criterion, using upper and lower limit principle, obtained the analytic solution for the resistance of a infinite long probe with oval cross section in a infinite space of soft clay. Results show that, sounding along the short axis, the influence of surface roughness on the probe resistance reduces with the decreasing of short to long axial ratio. With an axial ratio of 0.3~0.5,the penetration resistance difference is reduced to 9%~14%.