The effects of degree of coherence in optical lithography using the dummy diffraction mask are investigated by simulation and experiment. Influences on resolution and depth of focus of the repeated and isolated patterns are simulated for the several coherence factors of the illumination. The process margins are also simulated through the exposure vs. defocus diagram. The Lithographic performances of the complex periodic patterns are experimented. As degree of coherence (DOC) decreases, the contrast benefit is increased for Line and space(L/S) patterns size less than 0.5 lambda/NA where NA is the numerical aperture, and the depth of focus(DOF) benefit is increased for L/S patterns size less than 0.65 lambda/NA. The smaller L/S patterns size is, the greater the influence of DOC is. In the isolated patterns of sub-resolution type, better lithographic performances could be obtained for the smaller coherence factor.