Sexual size dimorphism in bird species has been claimed to reduce intersexual food competition and expand the species' overall feeding niche, yet quantitative diet information is known for only a few species. Lanyu scops owl (Otus elegans), a nocturnal insectivore on Lanyu (also called Orchid Island, southeast of Taiwan), has larger females than males. Lanyu scops owls rarely produce pellets, rendering quantification of its diet particularly difficult. We studied the diet of wild Lanyu scops owls through fecal analysis and verified the reliability of our method with a captive owl. Our results showed that fecal analysis was more reliable than observation alone because observations often missed small prey items and resulted in a much higher proportion of prey being unidentified. The feces of 81 Lanyu scops owls showed that orthopterans, beetles, moths, spiders, and centipedes constituted 67% of the total occurrences and >80% of the volume in their diet. Between the breeding and nonbreeding season, owl diets differed slightly in proportion but not in food types. Intersexual diet differences existed for both the breeding and nonbreeding seasons. Adult males ate a higher proportion of aerial insects than did adult females, which is consistent with the prediction that smaller males are more agile than larger females in capturing aerial insects. Male owls ate a lower proportion of terrestrial arthropods than did both females and juveniles in the breeding season, perhaps due to differential use of forest strata when foraging by male and female owls. Fecal analysis provides more reliable information on the diet of insectivorous owls than observations alone and is a non-invasive option for studying the diet of rare or threatened owls.