The structure properties of LaNi
3.5Al
1.5 have been investigated by the density functional theory using the full-potential linearized augmented plane wave (FLAPW) method with the generalized gradient approximation (GGA). The elastic constants of LaNi
3.5Al
1.5 are determined by a least-squares fitting of the calculated binding energy with a generalized two dimensional cubic function of lattice parameter
a and
c. The results show that the calculated lattice parameters (
a=b=0.5092 nm and
c=0.4214 nm) are in good agreement with the corresponding experimental data, that Al atoms prefer to substitute Ni atoms in the 3g sites and tend to disperse in middle planes. For LaNi
3.5Al
1.5, the bulk modulus (123.1 GPa), the shear modulus (68.6 GPa) and the values of the elastic constants C
11+C
12 (296.2 GPa), C
13 (76.4 GPa) and C
33 (210.5 GPa) are worked out for the first time. The ratio of c/a tends to increase from 0.790 (LaNi
5) to 0.804 (LaNi
4Al) and 0.827 (LaNi
3.5Al
1.5), which indicates that the LaNi
3.5Al
1.5 compound is observed to change less anisotropic with the substitution of Al atoms than the host alloy.
