This paper proposes a numerical calculation method to predict solidification characteristics of paraffin oil droplets (tetradecane, CH3(CH2)12CH3, melting point is 5.8°C, diameter is 4∼6mm) ascending in a cold water solution (mixture of ethylene glycol [CH2OH·CH2OH] and water, temperature is -2∼-10°C) by buoyancy. The oil droplets start to ascend with several initial velocities and initial temperatures (10∼60°C) in the water solution. The oil droplets are decelerated by hydraulic resistance of the water solution. The water solution flows vertically downward at a low velocity of 0.1 m/s. The temperature of the oil droplet decreases due to direct-contact heat transfer with the cold water solution, and the oil droplet starts to solidify when its surface temperature reaches the freezing point. The solidification rate of the oil droplet is calculated based on a combination of flow behavior and solidification characteristics of the oil droplet. As a result, the motion of the oil droplet, the completion height and time for solidification are determined. The parameters used are the diameter, initial velocity and initial temperature of the oil droplet, and temperature and velocity of water solution.