Abstract: In order to reduce the influence of surface roughness on the test result, rectangular section probe of T-bar test is presented. Assuming that soft clay strength obeys the Tresca yield condition and the resistance of rectangular probe is equal to the sum of that of single level and vertical plate without thickness, the approximate solution of the coefficient of the resistance of rectangular section probes is obtained. A comparison between rectangle section probe and oval section probe is made in terms of mechanical characteristics, deformation and strength. Results show that rectangular probe is better than oval cross section probes in those aspects. Rectangular cross section probe is a kind of applicable test probe.