Dielectric behavior of a commercially available sunflower oil based natural ester is compared with Shell Diala mineral oil and presented in this paper. Sequence of Frequency Domain Spectroscopy (FDS) and Polarisation Depolarisation Current (PDC) measurements are performed on oil samples with different ageing and temperature conditions. The observed response patterns show that real capacitance and AC conductivity is dispersed in low frequency region of FDS. Moreover, measured polarisation current monotonically decreases over measuring time. This study has identified that this behaviour is due to the electrode polarisation phenomenon, which is affected by temperature and ageing. This paper also investigates the gassing behavior of natural ester under transformer operating environment. Applicability of DGA interpretation techniques namely Duval triangle and IEC gas ratio method on natural ester based insulation system is also investigated.