The geometries, stabilities and electronic properties of PtnAl (n=18) clusters are calculated using density functional theory at BPW91/LANL2DZ level. The stabilities of the ground states of PtnAl (n=18) clusters are discussed by means of the binding energy, the second difference in energy and energy gaps, and the magnetic properties. Mulliken charges are studied. The growth patterns for different sized PtnAl (n=18) clusters are of Al-substituted Ptn+1 clusters and they keep a similar framework of the most stable Ptn+1 clusters except Pt2Al. Al atoms in the ground state PtnAl isomer tend to occupy the most highly coordinated positions. The analyses of stabilities show that PtAl and Pt4Al are more stable than other clusters. Mulliken population analysis shows that charges are transferred from Al atoms to Pt atoms, which indicates that Al atom acts as electron donor in all PtnAl clusters. The analysis of magnetic property shows that doping an Al atom reduces the average atomic magnetic moment of the host Pd cluster. Pt-rich clusters which have a strong nonlinear optical effect and are easy to polarize by external electromagnetic field.