A new structure of a micro-ring resonator for label-free biosensing is proposed. The structure includes sidewall-grating Si waveguide and periodical side-blocks that can enhance the light-matter interaction. From the electromagnetic simulations, the proposed structure exhibits a four-fold improvement on the sensitivity compared with the conventional structure. Moreover, the quality factor of the proposed structure is not degraded from that of the conventional structure. The improved sensitivity is promising for the detection of nanoparticles that can be applied to the environmental field and clinical diagnostics.