"Double flower" is a popular trait in ornamental plants. Cyclamen persicum, one of the major pot plants in Japan and European countries, displays various types of double flowers. In a previous study, the petaloid-stamen type of double-flowered cyclamen formed more true stamens instead of petaloid stamens at constant 20°C than at 15°C. Although there are reports that temperature affects stamen development in several plant species, the effects of alternating temperature on stamen development and floral morphogenesis in the formation of double flowers is not clear. Therefore, this study was conducted to clarify the effect of alternating temperature on floral characteristics in petaloid-stamen-type double-flowered cyclamen. Five plants of petaloid-stamen-type clonal double-flowered cyclamen 'Wink Pink II' were subjected to one of each temperature treatment in this study. The plants were maintained at constant 15°C or alternating 20/10°C (day, 06:00-18:00 h/night, 18:00-06:00 h), under 70% relative humidity and the natural photoperiod in a phytotron. Completely opened flowers were sampled every week from 1 January to 19 March 2015, and the number of flowers, form of petaloid stamens, length of flower stalks, and length and width of original petals were investigated. The accumulated number of flowers and petal length were greater at constant 15°C than at alternating 20/10°C, but the other floral characteristics were similar between treatments. Alternating temperatures therefore affected part of floral morphogenesis in petaloid-stamen-type double-flowered cyclamen. This information helps to decide the cultivation temperature for efficient and stable commercial production of double-flowered cyclamen.