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A cone is used to represent the probable track of the center of a tropical cyclone, which is formed by enclosing the area swept out by a set of circles along the forecast track (at 24, 48 hours, etc). The size of each circle is set so that the entire track of the tropical cyclone can be expected to remain within the cone roughly 60% to 70% of the time. Based on forecasts over the previous 4 years (from 2004 to 2007), the relationship between tropical cyclone track forecast error and their forecast movement speed and direction is used in classification, and the probability circle radiuses are calculated by means of the statistical method as for the forecast valid time of 24, 48, 72 hours, respectively. The old data used operationally since 2007 are replaced with the new results calculated for the western North Pacific basin in order to reduce the forecast uncertainty. Meantime, the subjective forecasts in the recent five years are analyzed in terms of the modified probability circle radius. The characteristics of forecast errors and the causes of great errors in some tropical cyclone forecasting cases are also given from 2004 to 2007. Possibility of improving the calculation method of radius is also discussed.