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.