This paper presents the variation of some selected parameters of a co-axial probe fed rectangular microstrip antenna as a function of Radome thickness and dielectric constant. Better range of radome thickness at 2300MHz in S-Band is found to be 0.1-0.6 mm with superstrate dielectric constant of 2.2. To give a maximum VSWR of 3:1. The better range of superstrate (a) Dielectric constant is up to 5 at 0.1mm thickness. (b) Dielectric constant is up to 3 at 0.2-0.3mm thickness (c) Dielectric constant is up to 2 at 0.3-0.6mm thickness at a maximum VSWR of 3:1. Some characteristics of co-axial fed rectangular microstrip antenna as a function of dielectric constant and thickness of a radome have been evaluated using computer simulation using IE3D software from Zeland. The antenna characteristics are investigated includes frequency shift, VSWR variation, gain, beam widths in E &H- Planes respectively.