Resonance frequency of an Equilateral triangular microstrip antenna (ETMSA) depends on its height. This property of the ETMSA is used effectively to realize isosceles TMSAs with the same height, thus resonating at the same frequency. The isosceles TMSA elements are designed as non-identical elements based on its area and hence gain variation with respect to three different standard amplitude distributions. An analysis of the antenna arrays using these non-identical elements to realize tapering in the E-plane for reduction of First sidelobe level (FSLL) has been presented. To verify the proposed concept, one TMSA array using eight numbers of non-identical triangular elements designed at 6 GHz is fabricated and tested. A reasonably good agreement between the simulated and measured results validates the proposed concept.