Rectangular photoresist gratings can be fabricated by taking advantage of the nonlinearity of photoresist. For rectangular gratings the task of controlling groove profile is simplified to controlling the duty cycle and groove depth. By monitoring diffraction intensity of latent image in photoresist in real time, the optimal exposure dose can be determined. Then, by observing characteristic behavior of the development-monitoring curve, the development process can be stopped at the moment when photoresist at the groove troughs is completely etched away. Therefore, rectangular grooves with clean troughs can be obtained; meanwhile the groove depth is kept equal to the initial photoresist thickness. Further development will reduce the duty cycle. It has been proven reliable by experiments and theoretical analysis. For gratings with 1200 lp/mm groove spacing, the groove depth up to 1 μm has been controlled accurately in authors' present fabrication, and the duty cycle value obtained is in the range 0.2 to 0.6. The experimental results have also demonstrated that good controllability of groove profiles depends on good fringe stability of the laser interference patterns.