This study investigated the ecological characteristics of the box jelly, Carybdea rastonii, in the waters off eastern Taiwan from 2007 to 2009. We started from field net sampling and some basic morphometric analysis to study its population dynamics, and continued with gut-content analysis, digestion rate and respiration rate experiments to estimate on the energy budget of C. rastonii, as well as its ecological significance in this region. We found the spots around the margin of pedelium through all samples, suggesting a stable morphological character of this species. Some specific behaviors were also observed, including the vertical movement after feeding and resting behavior (either lying on the bottom or drifting with the flow) during nighttime. Diagonal bell width (DBW) of C. rastonii was used as the size parameter, ranging from 4.17 to 46.39 mm in the collected samples, and was of significant positive-correlation with volume, wet weight, dry weight, and the length of statolith, suggesting DBW as a good indicator in age-determine. Weekly population structures indicated at least two peaks of strobilation occurring during the sampling period in 2008. Further analysis on the potential impact from environmental perturbation, by comparing the values of DBW with the length of the specimens taken immediately before and after typhoon attack, suggested a potential starvation effect on the local population. Results from gut-content analysis indicated a high percentage of empty guts (63.09%) in the checked specimens, having crustacean as the most dominant prey (up to 68.79%), and a diet shift to more larval fish from larger individuals. Based on the results from gut-content analysis and digestion rates on different types of prey, the estimated population carbon intake of C. rastonii (at 28 ind m-3) was about 20.77 mg m-3 d-1. However, results from respiration rate experiments suggested a higher basic metabolic demand of the box jelly (up to 0.126£gmol O2 h-1 mgDW-1, at 27¢J), which is significantly higher than other scyphomedusae of similar temperature range. According to the need of metabolism, the demand of carbon intake was estimated at 39.40 to 47.77 mg C m-3 d-1, an amount that might be under estimated for this voracious predator. Based on the estimated high values from both feeding and physiological demand, compared to the much lower primary production rate of 7.6 mg C m-3 d-1 in the Kuroshio waters, we concluded that C. rastonii as an important predator in this region, especially during its blooming seasons.