Celosia cristata L. Kurume type is a quantitative short-day plant with entirely globular inflorescences that is popular as a cut flower in Japan. The photoperiod sensitivity limits production and this could be overcome by prolonging its seedling stage by long-day treatments. In this study, the effects of transplant age and long-day treatment at seedlings stage on cut flower quality of C. cristata Kurume type cv. Sakata-pride were investigated. Plug transplants raised under 15h (natural day length) or 24-h photoperiods in a greenhouse were transplanted into wooden containers when two, four or eight leaves had unfolded. Flower bud initiation had already occurred in transplants with eight unfolded leaves grown in natural but not in 24-h day length. Days to visible bud were prolonged by 24-h treatment. The cut flowers grown in 24h were longer than those grown in natural day length. The weight of cut flower and inflorescence decreased as transplant age increased. The inflorescence form changed from globular to flat as transplant age increased. The youngest transplants had the highest cut flower quality. These results suggest that in order to produce highly marketable quality cut flowers of C. cristata Kurume type, the plug transplants must be planted before 4 leaves unfold, and that prolonging the seedling stage may be difficult through long-day treatment.