This paper presents an 18-to-32-GHz ultra wideband (UWB) low-noise amplifier (LNA) in a bulk 0.13-μm CMOS technology. The LNA consisting of four stages exhibits a flat gain of 14.5 ± L.5 dB over the entire 18-to-32 GHz and a noise figure of 5.5 ± 0.4 dB at K-band (18 to 26.5 GHz). Moreover, the group delay variation is suppressed as low as 63.5 ± 6.5 ps over 21-26 GHz. The wideband characteristics of gain and group delay are achieved by a slope offset technique. The individual stages are designed, such that the transfer function of each stage shows its peak at different frequencies by adjusting the output resonance network. The pole-zero analysis is performed to validate the slope offset technique and to provide a practical design guideline. Due to the wideband performance of flat gain and group delay, the proposed LNA is suitable for UWB communication or radar applications that demand an instantaneously wide fractional bandwidth.