Introduction: On the basis of percent depth dose (PDD) calculation, the application of dosimetry in radiotherapy has an important role to play in reducing the chance of tumor recurrence. The aim of this study is to introduce a new formula for calculating the centeral axis percent depth doses of Cobalt-60 beam. Materials and Methods: In the present study, based on the British Journal of Radiology (BJR) table, nine new formulas are developed and evaluated for depths of 0.5 - 30 cm and fields of )4 4( × ) 45 45 ( × − cm2. To evaluate the agreement between the formulas and the table, the average of the absolute differences between the values was used and the formula with the least average was selected as the best fitted formula. The Microsoft Excel 2000 and the Datafit 8.0 soft wares were used to perform the calculations. Results: The results of this study indicated that one amongst the nine formulas gave a better agreement with the PDDs listed in the table of BJR. The new formula has two parts in terms of log (A/P). The first part as a linear function with the depth in the range of 0.5 to 5 cm and the other one as a second order polynomial with the depth in the range of 6 to 30 cm. The average of the differences between the tabulated and the calculated data using the formula ( Δ ) is equal to 0.3152. Discussion and Conclusion: Therefore, the calculated percent depth dose data based on this formula has a better ageement with the published data for Cobalt-60 source. This formula could be used to calculate the percent depth dose for the depths and the field sizes not listed in the BJR table.