Abstract: Object: To investigate the statistic relationship between 5 clock positions of CTDI₁₀₀ and stabilization of radiation. Method: Radiation meter and phantom are used to 12 time repeating tests on the radiation dose of a spiral CT at 5 Clock positions and then one-way ANOVA method is used to character the their difference significance of those 5 clock positions. Result: The relevant standard difference of measurements at 5 test clock positions are 0.33%(center), 0.73%(3 clock), 0.32%(6 clock), 1.00%(9 clock), 0.32%(12 clock) respectively; meanwhile the average of measurements at 3 clock is much close to that at 9 clock and no significant difference is found in one-way ANOVA test;Moreover the relative deviation between mean value of 12 Clock plus 6 Clock and 3 Clock is 0.5%. Conclusion: The tested spiral CT shows good radiation stability and 3 Clock position have equal effect with 9 clock's in CTDI₁₀₀ measuring, moreover, peripheral CTDI of that can be simplified except for non-acceptance test or operational condition test.