The 3D finite element model of FBAR was established using ANSYS to analyze the effects of spurious lateral modes caused by different shape and area size of the active region and by the different piezoelectric materials. The shapes of the active area were the square, circular and irregularpentagon. The area sizes were 10 000 μm2、15 625 μm2、22 500 μm2、30 625 μm2、40 000 μm2 and the piezoelectric materials were AlN and ZnO. The impedance characteristic curve and nonecircularity NC were obtained. FBARs with irregularpentagon active area had the most smooth impedance characteristic curve and the smallest value of NC is 0.015 1. The value of NC and the spurious lateral modes decreased with the the increase of the active area and the smallest value of NC was 0.005 5 with the area size being 40 000 μm2. Operating at the same frequency of 1.76 GHz, the smallest value of NC is 0.012 9 with ZnO being the piezoelectric material.