In this letter, a compact ultrawide stop-band frequency selective surface (FSS) with polarization-insensitive and higher level of angular stability property is presented. It consists of a very simple modified square-ring unit cell that exhibits excellent wideband rejection over the entire ultrawideband (UWB) 2.72-13 GHz range. For TE incident wave, the <-20 dB bandwidth is 67.41% (5.01-10.1 GHz), which is 18.38% more compared to the simple square-ring-type FSS. This structure with the smallest unit cell dimension (0.066λ0 × 0.066λ0 × 0.013λ0) compared to the earlier reported UWB FSSs is printed on a single-layer single-sided FR4 substrate. The design is flexible enough to tune the desired resonant frequency and the operating band by changing the length and width of the resonators. The design concept has been validated through proper measurement. This FSS can be used as a good reflector to enhance the gain of UWB antennas. © 2002-2011 IEEE.