Surface morphologies and microstructures of epitaxial (Pb1−xLax)TiO3 (PLT, x = 0.00, 0.08, 0.16, and 0.24) thin films grown on (001) MgO have been investigated using SEM, AFM, and TEM. Surface roughness of PLT films varies severely with La concentration. For 0.08La-PLT film, very smooth surface has been achieved with 9.3 Å of RMS roughness. 0.00La- and 0.08La-PLT films show 90° domain structure, and periodic dimension of the domain is reduced by La substitution. 0.16La- and 0.24La-PLT films show the presence of triangular grains, causing rough surface and poor crystal quality. However, they are distributed uniformly in 0.16La-PLT film while agglomerated in 0.24La-PLT film. Optical propagation losses of PLT films measured by prism coupling technique were 22.3, 6.0, 11.4, and 20.7 dB/cm for x = 0.00, 0.08, 0.16, and 0.24, respectively. Such a variation in optical losses seemed to be due to the surface morphology and abundance of domain boundaries that change continuously as a function of La concentration in epitaxial PLT thin films.