We present a 1.16-µm-radius disk cavity with ultrahigh quality (Q) factor by embedding the disk into a sunflower-type circular photonic crystal (CPC). The band gap of the CPC reduces the bending loss of the whispering-gallery mode of the disk, leading to a simulated Q of 10 7 , at least one order of magnitude higher than a bare disk of the same size. The design is experimentally verified with a record high loaded Q of 7.4 × 10 5 measured from an optimized device fabricated on a silicon-on-insulator substrate.OCIS Codes: 230.5750, 230.5298, 220.4241 Owing to their ultrahigh-quality-factor whispering-gallery modes (WGMs) slabs [12, 13], provide the possibility to reduce the bending loss because light propagation is inhibited inside the band gap. However, the discontinuous structure of the CGRs is unsuitable for integrating with the widely used membrane fabrication technology, and the conventional triangular lattice PC needs sophisticated hole adjustment [14] to match the circular symmetry of the disk. Alternatively, circular photonic crystal (CPC) is considered as a very promising approach because it simultaneously possesses continuous slab structure and isotropic band gap [15, 16]. In this letter we present a 1.16-µm-radius Si disk embedded in a sunflower-type[