In this paper, an ultra-wideband low noise amplifier (LNA) is presented using 0.18-μm RF CMOS technology. A complementary structure with series inductive peaking technique is introduced by using two inductor-loops to achieve flat gain. In the first and third stages, two complementary structures are utilized to achieve a high and flat gain, a wideband input impedance matching and a low noise figure. In addition, a shunt feedback is utilized to increase input impedance matching and stability. In the second stage, a common source structure is used as interstage, which increases the À3 dB bandwidth. The designed LNA presents a high and flat gain of 15.6-16.5 dB, an excellent input return loss better than À11 dB and a good NF of 2.2-3 dB in the frequency range of 3.1-10.6 GHz. Also, the proposed structure consumes 6.8 mW from a 0.8-V supply voltage and has an IIP3 of À4 dBm at 6.5 GHz. The proposed structure only occupies 0.59 mm 2 .