The role of light in sexual behavior of lepidopteran species has been studied for many years because of the wide variation in the habits of moths and butterflies. Light level is important for mating of butterflies, but information on the potential role of light on sexual behavior in nocturnal moths is scanty. This study was conducted to determine the relationship between light intensity and sexual behavior in a nocturnal moth species. The mating frequency and the hourly variations of both calling behavior and sex pheromone titer in the oriental tobacco budworm Helicoverpa assulta (Guenée) (Lepidoptera: Noctuidae) were determined under several light intensities (0.0, 0.5, 5.0, and 50.0 lux). We found that 1) high-intensity light (50.0 lux) suppressed calling behavior, pheromone production, and mating; 2) low-intensity light (0.5 lux) significantly promoted female calling and mating in less time (1 h) than in complete darkness (0.0 lux); and 3) no relationship existed between sex pheromone production and calling behavior. These results suggested that low-intensity light (0.5 lux) promoted mating. This study provides reliable background information for using sex pheromones in conjunction with light traps for integrated management of nocturnal moths.