This paper presents the results of AC electrical breakdown properties of coconut oil with and without Tertiary Butyl Hydroquinone (TBHQ) antioxidant. Ageing test of transformer oil was conducted according to the standard of IEC-61125 B at 120 oC for 72 h with the presence of copper as a catalyst. Experimental results showed that AC breakdown voltage of coconut oil was comparable to that of mineral oil. After ageing test, breakdown voltages of both coconut oil and mineral oil were decreased. The results also showed that breakdown voltage of aged coconut oil containing 0.5 wt% of TBHQ antioxidant (44.5 kV) was higher than those of aged coconut oil without TBHQ (40.8 kV) and aged mineral oil (24.6 kV). The breakdown voltage (V) of oils increased linearly with increasing the electrode gap (d), which was fitted by a line with a slope of EBD. The EBD of coconut oil (21.9 kV/mm) was higher than that of mineral oil (18.9 kV/mm). The presence of TBHQ (0.5 wt%) reduced the EBD of coconut oil of about 13 %. However, after ageing test, coconut oil containing TBHQ (0.5 wt%) showed a higher EBD in comparison with virgin coconut oil and mineral oil. TBHQ exhibited as an effective antioxidant for coconut oil under thermal oxidation conditions. Coconut oil had a lower lifetime index n than mineral oil (13.2 compared to 23.3).