  We present a 1.16-\mum-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 \times 10^5 measured from an optimized
device fabricated on a silicon-on-insulator substrate.
