In psychophysical experiments, the Delboeuf illusion was measured as a function of spatial parameters of the stimulus pattern. During the experiments, the stimulus shape, size, luminance, and the dimensions of inducing surround varied. Subjects were asked to change the size of the test part of the stimulus by adjusting its diameter to value that made the test part appear equal to the perceived size of the referent part. The difference in diameters between the test and referent parts of the stimulus, determined after the perceived equality was achieved, was considered to be the value of the illusion magnitude. The magnitude of the Delboeuf illusion was dependent on the type of the stimuli and their contrast: the filled circles with the luminance contrast yielded stronger illusion than the open circles and the stimuli with isoluminant colors. The magnitude of the illusion did not change noticeably with variations of the luminance of the stimulus, but diminished when the luminance of the stimulus approached the level of isoluminance with the background. The neurophysiological spatial filtering model, applied to the Delboeuf stimuli patterns, has provided computational results similar to the present experimental findings.