We develop and fabricate a 0.5-V rail-to-rail operational amplifier (op-amp) with ultralow-power operation in a 0.18-mu m standard complementary metal-oxide-semiconductor process. The op-amp has a two-stage structure that comprises a complementary input stage and a novel cross-coupled output stage. The cross-coupled output stage increases the transconductances of the metal-oxide-semiconductor field-effect transistors of the output stage without requiring an additional chip area. Hence, it increases the gain of the op-amp and drivability for a capacitive load. Our experimental results showed that the dc gain was 77 dB at the common-mode input voltage of 0.25 V with a supply voltage of 0.5 V. DC gains of more than 40 dB were obtained for common-mode input voltages ranging 50-450 mV. Furthermore, the unitygain frequency was 4.0 kHz and the phase margin was 56 degrees, with a capacitive load of 40 pF. The power consumption was 70 nW, including all bias circuits.