A small shorted square microstrip antenna with air dielectric (ε r = 1.001) substrate is presented. The probe fed square microstrip antenna incorporates a single shorting post of radius 0.6mm which significantly reduces the overall size or area by 82% from a conventional square microstrip antenna of same substrate and height. After connecting a shorting post 55% reduction in resonant frequency is achieved for the same square patch. The maximum gain and directivity of the shorted square microstrip antenna are 3.72 dBi and 4.68 dBi respectively at resonant frequency 3.29 GHz. Simulated return loss, gain, directivity and radiation patterns are shown.