A new technique for controlling the duty cycle (ratio of ridge width to period) of photoresist gratings made on top of a multilayer dielectric stack is studied. It is used in conjunction with the real-time monitoring technique during photoresist development, and based on the principle that the effective refractive index of the leaky mode that has a strong evanescent tail in the cladding varies with the duty cycle of the grating situated at the interface between the top dielectric layer and the cladding. By applying the relationship of the coupling angle and duty cycle, a desired duty cycle can be obtained by terminating development when the leaky mode is excited under a preset incident angle. The experimental results showed that the duty cycle of the resulting grating can be adjusted by changing the incident angle. The work preliminarily establishes the feasibility of the technique. Experimental setup and procedure are presented, and the error sources and their influences are discussed.