This paper presents the design, construction and testing of grounded Frequency Selective Surface (FSS) array as a diffuser for destroying millimeter wave coherence which is used to eliminate speckle in active millimeter wave imaging. To create stochastically independent illumination patterns we proposed a diffuser based on random phase distributions obtained by changing the incident frequency. The random phase diffuser was obtained by mixing up the phase relations between the cells of a deterministic function (e.g. beam splitter). The slot length of FSS is the main design parameter used to optimize the phase shifting properties of the array. The critical parameters of the diffuser array design, such as phase relation with slot lengths, losses, and bandwidth are discussed. We designed the FSS arrays with Finite Integral Technique, fabricated by etching technique and the characterized the S-parameters with a free space MVNA and measured the radiation patterns with a BWO in motorized setup.