The life history and population structure of the amphipod Synchelidium lenorostralum were studied monthly for one year in the sandy shore at Dolsando, southern Korea. The life-history pattern was annual and iteroparous, with two dominant recruitments in fall (October to December) and spring (March to May). The sex ratio was female-biased, especially during breeding periods. The maximum size of females was 10.8 mm and 1.4 times larger than that of males. The mean body length of adults and brood size were greater in the spring breeding period than in the fall. There was a positive relationship between the brood size and body length of ovigerous females. The mean egg volume of stage I was more than 0.04 mm³ in December and March, but in other months it was less than 0.03 mm³. Egg volume was positively related to body length of ovigerous females and increased significantly at the phase between stage I and II. However, no significant relationship between egg volume and brood size was found. At any given female size, egg volume was significantly higher in the fall breeding than in the spring, whereas brood size was not different between the two main breeding periods, suggesting higher reproductive effort during the fall breeding period.