We describe the design, fabrication and bench-study of mm-wave quasi optical phase shifter by using grounded Frequency Selective Surface (FSS) on Roger 4003C substrate. This paper provides the basis for an understanding of FSS-based free space phase delay realization by using a FSS array composed of two sub-arrays of different slot lengths. The simulation and measurement results at W-band demonstrate that the phase delay between arrays can be controlled by the difference of slot lengths of the two sub-arrays. The measured S-parameters of the sub-arrays at different positions on the array surface demonstrate the phase mixing properties of FSS sub-arrays. We measured 180 degree phase delay between two sub-arrays with slot length of 896 µm and 1076 µm with slot width of 400 µm at 91.61 GHz.