It has been demonstrated that the exaggeration of male sexual ornaments and the intensity of female mate preferences of a wild guppy population change over a period of several months. However, the factors that determine the short-term changes in male ornaments and female preferences remained unclear. In this study, we examined the effect of season on these short-term changes by measuring these traits in the same seasons of different years for a wild guppy population in Okinawa, Japan. We also compared the characteristics of the offspring in each collection term, as female guppies are known to have the ability to control offspring characteristics, such as brood size and sex ratios, depending on their mates' attractiveness. Results showed that the total lengths of the males changed seasonally; males in the summer were larger than those in the spring. In contrast, the size of orange spots in males and the intensity of female mating preferences differed in the same seasons of different years. Brood size and offspring body size in each term showed seasonal changes. However, offspring sex ratios exhibited different patterns in the same seasons of different years. Females produced female-biased broods when attractive males with large orange spots were rare. These results suggest that short-term changes in some traits of adult male and female guppies as well as offspring sex ratios may be not determined by seasonal factors, and that these traits may be interrelated.