We have investigated gain flattening of EDFA systems with or without a gain equalizer for WDM long-haul transmission using a re-circulating EDFA loop. Without a gain equalizer, gain variation as small as 2.9 dB was achieved over the 10-nm band of a 100 cascaded EDFA system by the inversion principle. With a gain equalizer based on all-fiber acousto-optic tunable filters, two different config-urations of EDFAs were tested. For a single-stage EDFA scheme, the 21-nm band has shown 3.8 dB of gain variation at 17.4 ∼ 20.3 dB of OSNRs after the 100the stage of EDFAs. For a dual-stage EDFA scheme, a wider bandwidth of 34 nm has shown 3.6-dB variation after 40 cascaded EDFAs.