Novel sub-wavelength silicon photonic waveguides for label-free sensors are demonstrated in this article. We use silicon-on-insulator (SOI) waveguides that consist of sub-wavelength grating (SWG) structures, where the waveguides are made of small silicon arrays (180 × 180 nm<sup>2</sup> rectangles with 60 nm gaps). They are used to form microring and Bragg grating resonators which measure the change of refractive index by monitoring the resonant wavelength shift. Due to the high surface contact area and low optical confinement of the proposed waveguide, the sensitivity (both bulk and surface) can be significantly increased. The bulk sensitivity of 580 nm/RIU for the microring and 610 nm/RIU for the Bragg grating are better than other recently published resonator sensors. Moreover, a standard biological sandwich assay demonstrates an enhanced surface sensitivity of 2050 pm/nm for both devices. Theoretical models and experimental results are investigated, indicating the predominant losses are from the water absorption at 1550 nm and scattering by sidewall roughness.