The probability of the presence of stacking faults in the p-form single crystals of syndiotactic polystyrene, which were grown isothermally from a dilute solution at a temperature ranging from 150 to 210 degrees C, was estimated from the mean half-breadth of the streaked reflections. The probability was also estimated by counting the number of the faults recorded in the dark-field and the high-resolution electron microscopic images. The probability showed weak dependence on crystallization temperature, with its maximum value at 165 degrees C. When the single crystals grown at 165 degrees C were annealed isothermally even at 260 degrees C, only a small decrease in the amount of stacking faults was detected. The molecular mechanism of and difficulty in eliminating the stacking faults by annealing were discussed on the basis of the structural model of the fault.