We propose an ultracompact parallel label-free biosensor based on concentric silicon-on-insulator microring resonators. Our numerical studies show that the proposed biosensor offers higher sensitivity and a larger detection area than conventional single-ring-based sensors. We analyze the detection sensitivity of the DNA hybridization by immobilizing the probe on top of the double concentric ring resonators and in the ring–ring air gap. For a concentric double-ring system with an area of $27.646{\mathrm{\mu m}}^2$ , the bulk detection sensitivity is $683\mathrm{nm}$ ${\mathrm{RIU}}^{-1}$ . The sensitive uniformity of the system related to the optical field distribution is also studied.