We demonstrate optical bistability in silicon using a high-Q (Q > 105) one-dimensional photonic crystal nanocavity at an extremely low 1.6 microW input power that is one tenth the previously reported value. Owing to the device's unique geometrical structure, light and heat efficiently confine in a very small region, enabling strong thermo-optic confinement. We also showed with numerical analyses that this device can operate at a speed of approximately 0.5 micros.