Geometries, stabilities, and polarizations of PdnAl(n=18) have been calculated using the density functional theory at BPW91/LANL2DZ level. The growth pattern for different sized PdnAl(n=18)clusters is of Al-substituted Pdn+1 clusters, which shows the similar configuration of most stable Pdn+1 clusters except that of Pd6Al and Pd8Al. Geometries of ground state PdnAl(n=18) clusters keep the same structures of PdnAl clusters except that of Pd2Aland Pd6Al+. Al atoms in the ground state PdnAl and PdnAl isomers tend to occupy the most highly coordinated position. Analysis of stabilities shows that Pd4, Pd3Al and Pd3Al are more stable than other clusters. Study of polarizations shows that Pd-rich clusters have a strong nonlinear optical effect and are easy to be polarized by external electromagnetic field.