The operational temperature range of thermosiphon is generally limited from the critical point to the triple point of the working fluid to maintain two-phase state. Thermosiphon with mixed working fluid has a potential to widen the operational temperature range. In this study, the physical behavior of mixed working fluid during the transient operation of thermosiphon was analyzed with temperature-mole fraction diagram. The condenser and the evaporator temperature variations were explained by the dew line and the bubble line of the mixture. It is encouraging that the thermosiphon operation commences early with larger fraction of high boiling point component, but the temperature gap between the condenser and the evaporator due to the separation of two components has a negative effect on the officient cool down process.