Effects of vernalization and photoperiod on inflorescence formation in radish (Rahpanus sativus L.) were investigated. Plants were exposed to photoperiods (8, 12, 16 or 24 hr) during or after low temperature treatment (6°C), respectively. We used a new index “flower formation index” to explain these effects on inflorescence formation in radish plants quantitatively. According to “flower formation index”, the inflorescence formation was suppressed under the condition of short photoperiods, and increased under the condition of long photoperiods after the low temperature treatment as compared with that during the low temperature treatment. The suppression of inflorescence formation markedly occurred at short photoperiod treatment of 20-30 days after 7 days low temperature treatment, but the tendency was decreased in 21 days low temperature treatment even if the short photoperiod treatment was extended until 40 days. It was supposed that the short photoperiod treatment could be available to suppress a part of the inflorescence formation in the time of low temperature induction in the field.