Drop structures and especially drop manholes are extensively employed in supercritical routes of sewer and drainage systems. Drop manholes remarkably affect hydraulic features of their downstream system, while their operation is dominated by the flow regime inside them. Poor hydraulic performance of these structures under Regime R2 was improved with the jet-breaker, yet its proper dimensions were needed to be precisely determined. In this paper, effects of the jet-breaker length, width, sagitta, and angle on drop manhole energy dissipation and air demand (as responses), under the inlet pipe 80% filling ration, were experimentally studied. The modern statistical design of experiment (DoE) methodology and dimensional analysis were utilized to design the experiments in accordance with the 24-1 IV fractional factorial design. Ten specific jet-breakers were examined and more than 135 tests were performed. The statistical analysis of the results revealed that both responses were significantly improved when the jet-breaker length and width were 2 and 1.4 times the inlet pipe diameter, respectively; its sagitta was equal to 0, and its angle was at 70°. The use of DoE resulted in 21% reduction in experimental runs, straightforward data analysis, and unbiased concluding.