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Line-edge roughness (LER) characterization is vital in the semiconductor industry to this day. This paper presents an EKEI (enhanced knife edge interferometry)-based system for photomask LER characterization. From the given EKEI-based system, the LER on the photomask pattern can be characterized. In this paper, the Fresnel (F) number-based geometrical optical model was developed for the LER-induced edge diffraction fringe simulation. The LER here was defined by three times the standard deviation of the edge profile on the photomask. Periodic rectangular functions were employed to characterize LER effects on the diffraction fringe patterns in simulation. The simulation-generated fringe pattern was then analyzed by cross-correlation. From the simulation result, the fringe pattern will get attenuated when adding the LER onto the line edge profile. Once the LER value increases, some of the high-order fringes vanish. A negative correlation between the similarity and the LER value was discovered. From the F number-based simulation model, the real photomask decorated with different LER values was designed and fabricated. After the photomask writing process, we used the EKEI system to scan the LER-programmed pattern for photomask LER characterization. By using the cross-correlation, the same trend was found in the experiment and simulation results. The fringe pattern from the edge was attenuated while the LER was added to the edge. The geometrical-based simulation and analysis model for photomask LER characterization was validated. From the simulation results, the fringe pattern from the edge was attenuated when adding LER on the edge. The experiment results show good agreement with the simulation ones. In sum, the proposed EKEI-based system has great potential to be utilized for on-machine photomask LER characterization and those defect inspection. The F number-based geometrical optical model also has the potential for diffraction-free pattern design and analysis.