This paper has demonstrated a coupling matrix based method used to synthesize a bandstop filter. This method is independent of change in source and load impedances. There are different approaches to implement bandstop filter among which frequency transformation method is more advantageous in terms of complexity. In this method, a bandstop filter is synthesized by frequency transformation from lowpass prototype to bandstop filter. MATLAB code is developed for frequency transformation and coupling matrix. The filter is designed on RT Duroid substrate with relative dielectric constant of 10.8 and thickness of 1.27 mm. The filter is designed and simulated on HFSS and response found is in good agreement with the response found from the mathematical model.