A W-band millimeter wave Artificial Magnetic Conductor (AMC) is realized by using a grounded Frequency Selective Surface (FSS) array of different dimensions of resonant slots. Unlike normal conductors, this new surface does not support propagating surface waves, and it reflects electromagnetic waves with no phase reversal. Geometrically it consists of slots on a metallic layer on top of metal backed Roger 4003CTM dielectric substrate. Commercial CST Microwave studio software is used for simulation FSS cells and the S-parameters are characterized with a free space VNA. The simulation and experimental results demonstrate that the structure behaves like an AMC in W-band.