A low power CMOS rail to rail op-amp. We realizes in SCNO 180nm technology under 1.8 power supply voltage. A constant trans-conductance is ensured for the whole commonmode input range. The class AB output stage also has a full voltage swing. The circuit provides a gain bandwidth of 16.9- MHz and a DC gain of 81.27 dB. The input transistors operate in weak inversion, which have big gm/Id value, so the power consumption is reduced.