The structural, electronic, dynamical and thermal properties of AuCu3-type LaSn3 and LaPb3 were investigated by using ab-initio pseudopotential method and a linear response scheme, within the generalized gradient approximation. The obtained atomic parameters are in good agreement with other available theoretical and experimental studies. We have found that calculated phonon modes are positive along the all synunetry directions, indicating that these materials are dynamically stable at the cubic structure. The obtained phonon energies of LaSn3 are in good agreement with Neutron scattering measurements. Obtained phonon modes are triply degenerated at the F point due to the metallic nature of these materials. However, at 300 K, specific heats of these compounds have been calculated 98.07 J/molK and 98.52 J/molK for LaSn3 and LaPb3 respectively.