Alternating current light emitting diodes (AC-LED) have attracted more and more attentions due to the advantage of free of alternate current (AC)/direct current (DC) converter in practical application. With the increase of input power, the rising junction temperature will have negative influence on the the luminous flux, the optical power, the lifetime and other parameters of LED. Therefore, the understanding of the principle of accurate temperature rise of AC-LED is the key to the chip design. The finite-element analysis software (FloEFD) is used to simulate the transient thermal properties of 1 W white AC-LED driven by DC and AC power respectively. The results show that the junction temperature of AC-LED driven by AC power, fluctuates periodically around that driven by DC power and the fluctuation frequency is the same as that of the input signal, but with an apparent phase shift. Meanwhile, the variation of junction temperature of AC-LED driven at different powers and frequencies shows that the average junction temperature and its fluctuation amplitude increases linearly with the input power at steady state, however, decreases with the increase of frequency.