According to the prediction of theories, both acoustic power flux distribution between the right and left acoustic ports emitted from LFM IDT is asymmetric. By means of optical diffraction, we have detected the distribution of acoustic fields excited by the LFM IDT. Three unapodized LFM IDT which have different parameters are studied. These transducers with aluminum electrodes were deposited on the piezoelectric substrates ofYZ‐LiNbO3. The experimental results show that the SAW power run radiated from the high frequency port of LFM IDT is found to be stronger than that from the low frequency one. The asymmetry increases with the number of the finger pairs, but decreases with the band width. In general, the asymmetry increases with the excited frequency but the relation is not monotonic. Some theoretical models are taken for numerical evaluation of the asymmetric distribution. The comparison between the theoretical curves and the experimental results are presented and discussed briefly. However, in the theoretical calculations the mode‐conversion from SAW into BAW induced by the interdigital electrodes, has not been taken into consideration.