The onset voltages of positive and negative coronas are theoretically predicted for SF6 - N2 mixtures. For positive onset, the method of calculation is based on the condition necessary for the onscl sucamer to propagate. For negative onset, the method of calculation is based on the criterion developed for the formation of repetitive negative corona pulses. The analysis is applied for rodplane and sphere-sphere gaps. The calculated onset voltages obtained in both positive and negative coronas agree well with those measured values within 4 10 YO.