In this paper, the gain flattening of a wideband Fabry-Perot cavity (FPC) antenna, using truncated partially reflecting surface (PRS) and slotted elliptical and rectangular shape artificial magnetic conductor (AMC) layers is proposed. FPC is fed using a metal plated microstrip antenna (MSA) which comprises three layers-elliptical slotted rectangular AMC-I layer, truncated PRS layer, and rectangular slotted elliptical AMC-II layer. AMC-II layer is designed complementary to AMC-I layer to obtain gain variation < 1dB over wide frequency band. Elliptical shaped AMC-II and truncated PRS reduce the reflected fields towards ground and thus improve front to back lobe ratio (F/B) and side lobe level (SLL). These layers resonate at higher frequency and thus reduce gain variation and couple electromagnetically with MSA and AMC-I layer to provide wide bandwidth (BW). The proposed antenna provides S11 < -10 dB, 17.2 dBi peak gain with gain variation < 1.2 dB over 5.7-6.4 GHz frequency band, which covers 5.725-5.875 GHz ISM and 5.9-6.4 GHz satellite uplink C band. Broadside radiation patterns have SLL < -19 dB, cross polarization (CPL) < -17 dB, and F/B > 20 dB with wide 3 dB gain BW of 15.2%. The overall antenna dimensions are 2.3λ0x2.75λ0x0.5λ0, where λ0 is the free space wavelength corresponding to 5.8 GHz, central frequency of ISM frequency band. The measured results of the prototype fabricated structure agree with simulation ones.