The strip and rib waveguides based on silicon-on-insulator(SOI) microring resonator structures are investigated by the Finite Difference Time Domain(FDTD) method. The theory of microring resonators used in biosensing was explored. The effects of geometric structure dimension on biosensor sensitivity were considered and analyzed. It is demonstrated that the sensitivity of strip waveguide is much higher than that of rib waveguide, which is proved by their mode field distributions, and the sensitivity coefficients of both strip and rib waveguides have the similar trend with the increase of waveguide width. Furthermore, the strip waveguide has the highest sensitivity coefficient when the cross section is square, whereas that of rib waveguide corresponds to a not totally symmetrical geometry. The maximum sensitivity of strip waveguide is 172.3 nm/RIU when the cross section is fully symmetry.