This case study presents compressor high speed coupling failure where torsional natural frequency is excited. The mature unit had operated successfully for more than 15 years with no changes that would affect fundamental natural frequencies within compressor train until a series of compromising events created gears degradation which triggered natural frequencies excitation. Results are presented herein of the evidence of the failure mode of the couplings which show fatigue fractures leading to catastrophic failure due to high alternating torsional stress.