The design of an improved CMOS wideband Low Noise Amplifier (LNA) is presented. The bandwidth range is between 1 —5GHz, and the LNA is able to be used in the devices for cellular communication system and wireless local area network. The LNA comprises two stages. The first stage circuit is able to transform the signal received from the antenna or fitter into differential signals. It is able to reduce the number of discrete components of receiver system by integrating the conversion circuit into the LNA. The second stage contains common-source and common-gate MOS transistors. This stage provides an enough power gain and a good output matching for signals. The performance of power gain becomes better by adding a matching inductance between two transistors of second stage circuit. The improved CMOS LNA is designed and simulated with TSMC 0.18μm technology by using ADS software. Results illustrate that the noise figure of the LNA is below 2.3dB in the range of 1—5GHz, the gain for entire bandwidth is 14dB and the scattering parameters of S₁₁ and S₂₂ are all less than -10dB, having good input and output matches of the low noise amplifier.