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In this study, the corona onset voltage (COV) of bipolar bare overhead bundle conductors for transmission lines has been investigated using numerical calculations and gas discharge theory. First, an accurate numerical method, the charge simulation method, was used to calculate the electric field distribution in the vicinity of bare sub-conductors in bundles. Then, the electric field values and a criterion from the gas discharge theory were used to evaluate the onset voltage for monopolar positive or negative coronas on the bare sub-conductors. The monopolar criteria for single conductors have been further developed for use in bipolar bundle conductors with a rational demonstration. The bipolar COVs obtained are in good agreement with the literature data for laboratory models and full-scale test lines. Furthermore, some numerical results for the COVs influenced by the different effect factors of the bundle conductors have also been discussed.