This paper investigates the supercooling phenomenon and the freezing probability of water inside horizontal cylinders during a cooling process of thermal storage. The effect of such factors as cooling rate, inside diameter of cylinders and mass of different nucleation agents is experimentally examined. By measuring the time variations of temperature profiles at various locations inside the test cylinder and the photography observation at the time of nucleation, the supercooling period of water and the extent of dendritic ice growth is presented. The probability curves of initial appearance of dendritic ice as a function of various factors are characterized. Besides, a simple model to predict the critical cooling rate for dendritic ice blockage and supercooling period for container water is proposed. By incorporating the experimental results and the theoretical predictions, a correlation of supercooling period for water inside horizontal cylinders has also been developed.